U.S. patent application number 13/405012 was filed with the patent office on 2012-08-30 for magnetically coupled system for mixing.
This patent application is currently assigned to ALGENOL BIOFUELS INC.. Invention is credited to Oliver Ashley, Joseph Katz, Edward Legere, Edwin Malkiel, George Meichel, Harlan Miller, III, Paul Petersen, Jason Ward, R. Paul Woods.
Application Number | 20120220027 13/405012 |
Document ID | / |
Family ID | 46719243 |
Filed Date | 2012-08-30 |
United States Patent
Application |
20120220027 |
Kind Code |
A1 |
Miller, III; Harlan ; et
al. |
August 30, 2012 |
Magnetically Coupled System For Mixing
Abstract
The invention provides a mixing system comprising a magnetically
coupled drive system and a foil for cultivating algae, or
cyanobacteria, in an open or enclosed vessel. The invention
provides effective mixing, low energy usage, low capital
expenditure, and ease of drive system component maintenance while
maintaining the integrity of a sealed mixing vessel.
Inventors: |
Miller, III; Harlan; (Fort
Myers, FL) ; Meichel; George; (Fort Myers, FL)
; Legere; Edward; (Lake Worth, FL) ; Malkiel;
Edwin; (Naples, FL) ; Woods; R. Paul; (Naples,
FL) ; Ashley; Oliver; (Fort Myers, FL) ; Katz;
Joseph; (Baltimore, MD) ; Ward; Jason; (Fort
Myers, FL) ; Petersen; Paul; (West Palm Beach,
FL) |
Assignee: |
ALGENOL BIOFUELS INC.
Bonita Springs
FL
|
Family ID: |
46719243 |
Appl. No.: |
13/405012 |
Filed: |
February 24, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61447004 |
Feb 25, 2011 |
|
|
|
61575644 |
Aug 24, 2011 |
|
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|
Current U.S.
Class: |
435/292.1 ;
366/262; 366/273; 422/225; 435/289.1 |
Current CPC
Class: |
C12M 21/02 20130101;
B01F 15/00545 20130101; B01F 13/08 20130101; B01F 13/0827 20130101;
B01F 13/0015 20130101; C12M 27/02 20130101 |
Class at
Publication: |
435/292.1 ;
366/273; 366/262; 435/289.1; 422/225 |
International
Class: |
C12M 1/02 20060101
C12M001/02; B01J 19/18 20060101 B01J019/18; C12M 1/42 20060101
C12M001/42; B01F 13/08 20060101 B01F013/08; B01F 5/12 20060101
B01F005/12 |
Claims
1. A magnetically coupled mixing system comprising: a mixing
vessel; a liquid disposed within the mixing vessel; a drive
conduit; a drive element disposed within the drive conduit and
adapted to move in a longitudinal direction within the drive
conduit; a follower member having a first distal end and a second
distal end, wherein the follower member is disposed around the
perimeter of at least a portion of the drive conduit and is adapted
to move longitudinally along the drive conduit; a magnetic follower
element disposed within the follower member, wherein the magnetic
follower element is adapted to couple magnetically with the drive
element and is proximally disposed outside the drive conduit; a
foil having a surface shaped or configured to provide hydrodynamic
lift, wherein the foil is disposed at least partially in the
liquid, and wherein vertical mixing of the liquid is achieved by
movement of the foil along a linear path in the mixing vessel; and
a support member connecting the foil and the follower member.
2. The magnetically coupled mixing system of claim 1 further
comprising drive fluid contained within the drive conduit and a
pump in communication with the drive conduit, wherein the pump is
adapted to move the drive fluid and the drive element within the
drive conduit.
3. The magnetically coupled mixing system of claim 2 wherein the
drive fluid is air, water, mineral oil, polyethylene glycol or
hydraulic fluid.
4. The magnetically coupled mixing system of claim 2 wherein the
pump is a reversible flow pump.
5. The magnetically coupled mixing system of claim 1 wherein the
mixing vessel is an enclosed vessel, an open vessel, a reactor, a
bioreactor, a photobioreactor or an open pond.
6. The magnetically coupled mixing system of claim 1 wherein the
liquid mixture comprises algae and water.
7. The magnetically coupled mixing system of claim 1 wherein the
drive conduit is disposed inside the reactor vessel.
8. The magnetically coupled mixing system of claim 1 wherein the
drive conduit is disposed at least partially within the liquid
mixture.
9. The magnetically coupled mixing system of claim 1 further
comprising a blocking element disposed within the drive conduit,
wherein the blocking element is adapted to restrict the movement of
the drive element within a desired range within the drive
conduit.
10. The magnetically coupled mixing system of claim 1 wherein the
drive element is ferromagnetic or magnetic.
11. The magnetically coupled mixing system of claim 1 wherein the
follower member comprises a hollow elongate tubular enclosure.
12. The magnetically coupled mixing system of claim 1 wherein the
magnetic follower element is adapted to move longitudinally within
the follower member.
13. The magnetically coupled mixing system of claim 1 further
comprising a flotation member and a support member connecting the
flotation member with the follower member or the foil.
14. The magnetically coupled mixing system of claim 1 wherein the
foil comprises a cambered surface.
15. The magnetically coupled mixing system of claim 1 wherein the
foil is pivotally attached to a support member, such that the angle
of attack of the foil variable and selectively adjustable.
16. The magnetically coupled mixing system of claim 1 wherein the
mixing system mixes only a portion of the depth of the liquid
disposed within the mixing vessel.
17. The magnetically coupled mixing system of claim 1 wherein the
mixing system mixes only a portion of the length of the liquid
disposed within the mixing vessel.
18. A system for the production of a target molecule or
accumulation of biomass comprising: a suspension of cells in a
liquid, wherein the cells are capable of producing a target
molecule or accumulating biomass; and a foil moving by magnetic
means in a linear direction in the liquid and producing vertical
mixing of the suspension; wherein production of the target molecule
or accumulation of biomass is greater in the presence of the moving
foil than in the absence of the moving foil; and wherein the system
is a bioreactor.
19. A magnetically coupled mixing system comprising: a mixing
vessel; a liquid disposed within the mixing vessel; a drive
conduit; a drive element disposed within the drive conduit and
adapted to move in a longitudinal direction within the drive
conduit; and a magnetic follower element disposed around the
perimeter of at least a portion of the drive conduit and adapted to
move longitudinally along the drive conduit, wherein the magnetic
follower element is adapted to couple magnetically with the drive
element and is proximally disposed outside the drive conduit.
20. A system for the production of a target molecule or
accumulation of biomass comprising: a suspension of cells in a
liquid, wherein the cells are capable of producing a target
molecule or accumulating biomass and a foil moving by magnetic
means in a linear direction in the liquid and producing vertical
mixing of the suspension; wherein the total cost of mixing per unit
weight of the target molecule or biomass produced is lower using
the foil to induce vertical mixing than using a paddlewheel mixer;
and wherein the system is a bioreactor
21. The magnetically coupled mixing system of claim 19 further
comprising a hose adapted to sparge gas into the liquid disposed
within the mixing vessel, wherein the hose comprises a first end
attached to the magnetic follower element and a second end.
22. The magnetically coupled mixing system of claim 21 wherein the
second end of the hose is attached to the mixing vessel.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Patent Application No. 61/447,004, filed Feb. 25, 2011, and U.S.
Provisional Patent Application No. 61/575,644, filed Aug. 24, 2011,
the disclosures of each of which are incorporated herein by
reference.
BACKGROUND
[0002] The present invention relates generally to mixing systems
for use in enclosed vessels, such as rigid or flexible enclosures,
or open vessels, such as pond systems, which may serve as reactors,
bioreactors or photobioreactors. Systems in accordance with the
present invention may be used to cultivate algae and other
microorganisms in water for purposes such as producing biofuels,
bulk chemicals, pharmaceutical compounds or other products or
treating wastewater.
[0003] High density, high pigment aqueous algae cultures require
mixing to evenly distribute nutrients to microorganisms in the
culture and to ensure that the microorganisms in the culture are
cyclically exposed to light needed for photosynthesis. One of the
key challenges for commercial-scale mixing systems is to minimize
the use of energy and capital expense while providing optimal
production conditions.
[0004] Large, open pond systems typically use large paddle wheel
mixers to move water around a raceway, but paddle wheel mixers are
inefficient and require significant energy inputs, which may be
cost-prohibitive for use in cultivation of microorganisms for the
production of biofuel or other commodities. In addition, paddle
wheel mixers are designed to move water in a path of horizontal
flow and do not effectively move algae in a vertical plane, which
is needed to ensure even exposure of the algae to light at the
surface of the aqueous culture.
[0005] In "Biotechnology of Algal Biomass Production: A Review of
Systems for. Outdoor Mass Culture," Journal of Applied Phycology 5:
593-604 (1993), Chaumont reviews mixing techniques proposed for use
with algae cultures, including forcing culture through a slit in a
board dragged through an open pond;. "a mixing system consisting of
a continuous flume containing arrays of foils similar in design to
segments of airplane wings"; airlift; injectors; propellers; pump
and gravity flow devices using natural energy sources; open pond
loop "raceways" incorporating paddlewheel stirring devices; and
sloped ponds and other cultivation units having parallel troughs or
baffles, for example.
[0006] In "Photobioreactors for Mass Cultivation of Algae,"
Bioresource Technology 99: 4021-4028 (2008), Ugwu et al. note that
inefficient stirring mechanisms in open cultivation systems yield
poor mass transfer rates that result in low biomass
productivity.
[0007] Vertical photobioreactor systems use pumps, blowers or
compressed air to introduce rising air bubbles and produce
turbulent fluid motion in the aqueous algal culture for the purpose
of mixing. Horizontal photobioreactor systems typically use pumps
to circulate the culture and create turbulence in the aqueous algae
culture to provide mixing.
[0008] Ugwu et al. ("Photobioreactors for Mass Cultivation of
Algae") describe the use of air-pump, bubble column and airlift
systems to mix cultures in tubular and vertical-column
photobioreactors.
[0009] The effects and performance of mixing in vessels such as
bioreactors have also been investigated for numerous configurations
of other mixing elements, such as the combination of radial
impellers with axial up-pumping hydrofoils (Vrabel et al., "Mixing
in Large-Scale Vessels Stirred With Multiple Radial or Radial and
Axial Up-Pumping Impellers: Modelling and Measurements," Chemical
Engineering Science, Vol. 55, No. 23: 5881-5896 (2000)); a rotating
impeller in combination with glass tubes acting as baffle plates
(Ogbanna et al., "A Novel Internally Illuminated Stirred Tank
Photobioreactor for Large-Scale Cultivation of Photosynthetic
Cells," Journal of Fermentation and Bioengineering, Vol. 82, No.
1:61-67 (1996)); up-pumping impellers (Nienow et al., "The
Versatility of Up-Pumping Hydrofoil Agitators," Chemical
Engineering Research and Design, Vol. 82, No. 9: 1073-1081 (2004));
axial and mixed dual-impeller systems (Bouaifi et al., "Power
Consumption, Mixing Time and Homogenisation Energy in Dual-Impeller
Agitated Gas-Liquid Reactors," Chemical Engineering and Processing,
Vol. 40, No. 2: 87-95 (2001)); the combination of airlift with
hydrofoil impellers (Chisti et al., "Oxygen Transfer and Mixing in
Mechanically Agitated Airlift Bioreactors," Biochemical Engineering
Journal, Vol. 10, No. 2: 143-153 (2002)); and turbines,
down-pumping hydrofoils and up-pumping hydrofoils (Boon et al.,
"Comparing a Range of Impellers for `Stirring as Foam Disruption`,"
Biochemical Engineering Journal, Vol. 10, No. 3: 183-195
(2002)).
[0010] In "A Simple Algal Production System Designed to Utilize the
Flashing Light Effect," Biotechnology and Bioengineering, Vol. XXV:
2319-2335 (1983) and in "High Algal Production Rates Achieved in a
Shallow Outdoor Flume," Biotechnology and Bioengineering, Vol.
XXVIII: 191-197 (1986), Laws et al. describe gains in solar energy
conversion efficiency and algae production yielded by emplacing
arrays of foils similar in design to airplane wings to create
vortices and systematic mixing in an algal culture flume.
[0011] Many of these methods provide mixing regimes for
high-density algae cultures but consume too much energy to be cost
effective in the production of biofuel, bulk chemicals or other
commodities on an industrial scale. The required energy inputs for
such methods and configurations exceed the energy yield that can be
produced by the algae culture in the form of, for example, biofuel.
Accordingly, a need exists for a mixing system that provides
sufficient mixing and gas transfer for optimal production of
biofuel and other materials while maintaining acceptable energy
consumption in the context of operating costs for the reactor
system and minimizing capital expense.
[0012] In addition, a need exists to provide effective mixing and
gas transfer in vessels such as bioreactors and photobioreactors
while maintaining structural integrity of the vessel, minimizing
risk of contamination of the contents of the vessel, minimizing
exposure of pumps and other mixing drive system components to
corrosive agents in the vessel and facilitating ease of maintaining
the drive system components. Various mixing apparatuses rely on the
use of rotary impellers and similar elements that are not
physically connected to a drive motor but instead are driven by
magnetic coupling.
[0013] U.S. Pat. No. 7,824,904 (Dimanshteyn for "Photobioreactors
for Production of Algae and Methods Therefor") discloses mixing a
liquid microbial culture using a rotary or oscillatory system
comprising one or more motors, one or more shafts connected to the
one or more motors and a plurality of mixing blades attached to the
one or more shafts.
[0014] U.S. Pat. Appl. Pub. No. 2009/0035856 (Galliher et al. for
"Continuous Perfusion Bioreactor System") discloses vessels such as
a disposable, collapsible bag having an integrated
magnetically-driven rotating impeller that provides mixing for cell
culture, cell containment, bioreactor and/or pharmaceutical
manufacturing systems.
[0015] U.S. Pat. Appl. Pub. No. 2009/0130757 (Terentiev for
"Bioreactor With Mixer and Sparger") discloses a bioreactor that
comprises an impeller positioned within an interior compartment of
the vessel that is rotated by way of a magnetic coupling.
[0016] U.S. Pat. Appl. Pub. No. 2011/0003366 (Zeikus for "Methods
of Using Pneumatic Bioreactors") discloses a pneumatic bioreactor
containing a fluid to be mixed that includes a floating impeller
that rises in the fluid as gas bubbles carry it upward to the
surface and falls when the gas is then vented, wherein the mixing
speed is controlled with electromagnets in the vessel acting upon
magnetic material in the impeller or its guides.
[0017] PCT Published Patent Application WO 2005/121310 (Johnson et
al. for "Creation of Shear in a Reactor") discloses the use of a
applying a magnetic field to a magnetically-activated element to
generate shear in a liquid sample.
[0018] U.S. Pat. Appl. Pub. No. 2009/0219780 (Castillo et al. for
"Mixing System Including a Flexible Bag, Specific Flexible Bag and
Locating System for the Mixing System") discloses a mixing system
comprising a flexible bag with a rotary magnetic impeller and an
alignment facilitation device adapted to facilitate alignment
between the magnetic impeller and a magnetic driver located
external to the system.
[0019] In "Design, Construction and Testing of Pilot Scale
Photobioreactor Subsystems," Master of Science (MS) Thesis, Ohio
University, Mechanical Engineering (Engineering and Technology),
2008, Mears describes the work of Tsygankov (2001) involving a
coaxial cylinder reactor in which two coaxial tubes are placed one
inside the other with algae fluid located in the annular space
between the surfaces of both tubes. Mears further describes the
reactor of Tsygankov incorporating a ferromagnetic ring in the
section containing the algae and applying a magnetic field to move
the ring back and forth, mixing the algae liquid.
[0020] In "Microbioreactors for Bioprocess Development,"Journal of
the Association for Laboratory Automation, Vol. 12, No. 3: 143-151
(2007), Zhang et al. describe the use of a magnetic stir bar to mix
a microbial solution in a cylindrical reactor chamber.
[0021] A need exists to incorporate a magnetic coupling drive
system with a mixing configuration that is effective in a
photobioreactor while maintaining the structural integrity of the
photobioreactor and ability to service the components of the drive
system without compromising the algae culture therein.
[0022] The above discussion includes both information known to the
art prior to the filing date and information forming part of the
present inventive disclosure. Inclusion of any statement in this
section, whether as a characterization of a published reference or
in a discussion of technical problems and their solutions, is not
to be taken as an admission that such statement is prior art.
SUMMARY OF INVENTION
[0023] An object of this invention is a magnetically coupled mixing
system adapted to provide vertical mixing in an open or enclosed
vessel while advantageously maintaining low energy usage
requirements.
[0024] A further object of this invention is a magnetically coupled
mixing system adapted to provide gas transfer in an open or
enclosed vessel while advantageously maintaining low energy usage
requirements.
[0025] A further object of this invention is a magnetically coupled
mixing system wherein components used to drive the mixing system
are located outside a sealed mixing vessel, permitting access
outside the sealed mixing vessel for greater ease of maintaining
the drive components, while maintaining the integrity of the sealed
mixing vessel.
[0026] Accordingly, this invention provides for a magnetically
coupled mixing system comprising a mixing vessel; a liquid mixture
disposed within the reactor vessel; a drive conduit; a drive
element disposed within the drive conduit and adapted to move in a
longitudinal direction within the drive conduit; a follower member
having a first distal end and a second distal end, wherein the
follower member is disposed around the perimeter of the drive
conduit and is adapted to move longitudinally along the drive
conduit; a magnetic follower element disposed within the follower
member, wherein the magnetic follower element is adapted to couple
magnetically with the drive element and is proximally disposed
outside the drive conduit; a foil having a surface shaped or
configured to provide hydrodynamic lift, wherein the foil is
disposed at least partially in the liquid mixture; and a support
member connecting the foil and the follower member.
[0027] This invention also provides for a magnetically coupled
mixing system comprising a mixing vessel; a liquid disposed within
the mixing vessel; a gas disposed within the mixing vessel; a drive
conduit; a drive element disposed within the drive conduit and
adapted to move in a longitudinal direction within the drive
conduit; a follower member having a first distal end and a second
distal end, wherein the follower member is disposed around the
perimeter of at least a portion of the drive conduit and is adapted
to move longitudinally along the drive conduit; a magnetic follower
element disposed within the follower member, wherein the magnetic
follower element is adapted to couple magnetically with the drive
element and is proximally disposed outside the drive conduit; and a
crossbar attached to the follower member, wherein the crossbar is
at least partially disposed in the liquid, the crossbar mixes the
liquid, the gas, or an interface between the liquid and the gas,
the crossbar has a surface shaped or configured to generate a
breaking wave front in the liquid, and the breaking wave front is
generated by movement of the crossbar along a linear path in the
mixing vessel.
[0028] This invention also provides for a magnetically coupled
mixing system comprising drive fluid contained within the drive
conduit and a pump in communication with the drive conduit, wherein
the pump is adapted to move the drive fluid and the drive element
within the drive conduit.
[0029] This invention also provides for a magnetically coupled
mixing system wherein the drive fluid is air, water, mineral oil or
polyethylene glycol.
[0030] This invention also provides for a magnetically coupled
mixing system wherein the drive fluid contains corrosion-inhibiting
agents.
[0031] This invention also provides for a magnetically coupled
mixing system wherein the pump is a reversible flow pump.
[0032] This invention also provides for a magnetically coupled
mixing system wherein the pump is a positive displacement pump or a
velocity pump.
[0033] This invention also provides for a magnetically coupled
mixing system wherein the pump is a diaphragm pump or a centrifugal
pump.
[0034] This invention also provides for a magnetically coupled
mixing system comprising a flow control valve.
[0035] This invention also provides for a magnetically coupled
mixing system wherein the pump is located outside of the mixing
vessel.
[0036] This invention also provides for a magnetically coupled
mixing system wherein the mixing vessel is an enclosed vessel, an
open vessel, a reactor, a bioreactor, a photobioreactor or an open
pond. In the context of a photobioreactor comprising cells in a
liquid suspension, the magnetic mixing system of the present
invention employing a linearly. moving foil provides gentle
vertical mixing which allows for particle distribution and movement
within alight field but without damaging cells. For situations
wherein the cells are producing a target molecule, the cost of the
energy consumed by the mixing system of the present invention is
less than the value of the target molecule, with energy consumed
and molecules produced averaged over the same time period. In a
preferred embodiment, the energy cost is 10% or less of the value
of the target molecules produced.
[0037] This invention also provides for a magnetically coupled
mixing system wherein the liquid mixture comprises algae and water.
This invention also provides for a magnetically coupled mixing
system that may be used in other commercial processes that require
low energy input and regular, gentle mixing in elongate reactors,
including but not limited to pharmaceutical cell culture, food
processing and waste water treatment. This invention also provides
for a magnetically driven skimmer that can economically remove
surface solids that accumulate in algal ponds and waste water. This
invention also provides for a magnetically driven foil that may be
used in a vapor phase to increase the efficiency of a solar
still.
[0038] This invention also provides for a magnetically coupled
mixing system wherein the drive conduit is disposed inside the
reactor vessel.
[0039] This invention also provides for a magnetically coupled
mixing system wherein the drive conduit is disposed at least
partially within the liquid mixture.
[0040] This invention also provides for a magnetically coupled
mixing system wherein the drive conduit is disposed outside the
reactor vessel.
[0041] This invention also provides for a magnetically coupled
mixing system comprising a blocking element disposed within the
drive conduit, wherein the blocking element is adapted to restrict
the movement of the drive element within a desired range within the
drive conduit.
[0042] This invention also provides for a magnetically coupled
mixing system comprising a longitudinal vane disposed on an inner
surface of the drive conduit such that channels adopted to permit
flow of the drive fluid are formed on the inner surface of the
drive conduit, wherein the channels are bounded by the surface of
the drive element, the surface of the longitudinal vane and the
inner surface of the elongate tubular hollow member.
[0043] This invention also provides for a magnetically coupled
mixing system comprising a longitudinal groove formed in an inner
surface of the drive conduit such that a channel adapted to permit
flow of the drive fluid is formed in the inner surface of the drive
conduit, wherein the channel are bounded by the surface of the
drive element and the surfaces of the groove formed in the inner
surface of the drive conduit.
[0044] This invention also provides for a magnetically coupled
mixing system wherein the drive element is ferromagnetic or
magnetic.
[0045] This invention also provides for a magnetically coupled
mixing system comprising materials that suppress corrosion or wear,
wherein the materials coat the drive element.
[0046] This invention also provides for a magnetically coupled
mixing system wherein the drive element is spherical or
cylindrical.
[0047] This invention also provides for a magnetically coupled
mixing system wherein the follower member comprises a hollow
elongate tubular enclosure.
[0048] This invention also provides for a magnetically coupled
mixing system wherein the magnetic follower element is adapted to
move longitudinally within the follower member.
[0049] This invention also provides for a magnetically coupled
mixing system comprising a flotation member and a support member
connecting the flotation member with the follower member or the
foil.
[0050] This invention also provides for a magnetically coupled
mixing system wherein the flotation member is configured to provide
surface mixing of the liquid mixture.
[0051] This invention also provides for a magnetically coupled
mixing system wherein the flotation member comprises a pontoon.
[0052] This invention also provides for a magnetically coupled
mixing system comprising a tracking member, wherein the tracking
member is proximal to a wall of the mixing vessel and is adapted to
prevent the foil and the flotation member from contacting the wall,
wherein the support member connects the foil, the follower member
and the tracking member.
[0053] This invention also provides for a magnetically coupled
mixing system comprising a carrier member in which the magnetic
follower element is contained, wherein the carrier member is
proximally disposed outside the drive conduit.
[0054] This invention also provides for a magnetically coupled
mixing system comprising a bumper element disposed within the
follower member.
[0055] This invention also provides for a magnetically coupled
mixing system comprising a flexible elongate tension member that
connects the foil or support member to the follower member.
[0056] This invention also provides for a magnetically coupled
mixing system wherein the foil is uncambered, has a quadrangular
planform shape and is configured at an angle of attack sufficient
to generate hydrodynamic lift and trailing vortices.
[0057] This invention also provides for a magnetically coupled
mixing system wherein the, foil comprises a cambered surface.
[0058] This invention also provides for a magnetically coupled
mixing system wherein the foil is substantially vertically oriented
and has a surface configured and angled to provide hydrodynamic
lift.
[0059] This invention also provides for a magnetically coupled
mixing system comprising an axle attached to a support member,
wherein the foil is rotatably mounted on the axle.
[0060] This invention also provides for a magnetically coupled
mixing system wherein the foil further comprises a weight or a
cavity embedded in the foil proximal to a trailing edge of the
foil.
[0061] This invention also provides for a magnetically coupled
mixing system wherein the foil further comprises a steering element
disposed on the surface of the foil proximal to a trailing edge of
the foil.
[0062] This invention also provides for a magnetically coupled
mixing system wherein the planform shape of the foil is bilaterally
symmetric and is triangular or quadrangular.
[0063] This invention also provides for a magnetically coupled
mixing system comprising a support member that is substantially
horizontally oriented, wherein a top edge of the support member is
cambered and is adapted to induce a hydraulic jump in the liquid
mixture.
[0064] This invention also provides for a magnetically coupled
mixing system comprising mixing structures attached to a horizontal
support member, wherein the mixing structures are configured to
stir the surface of the liquid mixture.
[0065] This invention also provides for a magnetically coupled
mixing system comprising a flexible dredging member attached to the
foil, the pontoon or a support member, wherein the flexible
dredging member is at least partially suspended in the liquid
mixture and is configured to induce vertical mixing of the liquid
mixture.
[0066] This invention also provides for a magnetically coupled
mixing system wherein the foil is pivotally attached to a support
member, such that the angle of attack of the foil is variable and
selectively adjustable.
[0067] This invention also provides for a magnetically coupled
mixing system comprising a poppet valve disposed inside the drive
element.
[0068] This invention also provides for a magnetically coupled
mixing system comprising a bypass conduit connected to the drive
conduit.
[0069] This invention also provides for a magnetically coupled
mixing system comprising a foil disposed only in gas.
[0070] This invention also provides for a magnetically coupled
mixing system comprising a gas sparging hose attached to the
follower member, the follower element, the foil or the support
member.
[0071] This invention also provides for a magnetically coupled
mixing system comprising a Venturi tube formed in the support
member, with one opening of the Venturi tube disposed above the
surface of the liquid and the opposite opening of the Venturi tube
disposed below the surface of the liquid.
[0072] This invention also provides for a magnetically coupled
mixing system comprising a vertically-oriented foil attached to a
horizontally-oriented foil.
[0073] This invention also provides for a magnetically coupled
mixing system wherein the mixing system mixes only a portion of the
depth of the liquid disposed within the mixing vessel.
[0074] This invention also provides for a magnetically coupled
mixing system wherein the, mixing system mixes only a portion of
the length of the liquid disposed within the mixing vessel.
[0075] This invention also provides for a magnetically coupled
mixing system comprising a mixing vessel; a liquid disposed within
the mixing vessel; a foil having a surface shaped or configured to
provide hydrodynamic lift, wherein the foil is disposed at least
partially in the liquid, and wherein vertical mixing of the liquid
is achieved by linear motion of the foil in the mixing vessel; a
cable attached to the foil; and a reversible motor adapted to pull
the cable.
[0076] This invention also provides for a magnetically coupled
mixing system comprising a drive conduit; a follower member having
a first distal end and a second distal end, wherein the follower
member is disposed around the perimeter of at least a portion of
the drive conduit and is adapted to move longitudinally along the
drive conduit, and wherein the cable is attached to the foil or the
follower member; and a support member connecting the foil and the
follower member.
[0077] This invention also provides for a system for achieving
vertical mixing within a liquid in a reactor comprising a foil
moving by magnetic means in a linear direction.
[0078] This invention also provides for a method to achieve
vertical mixing in a liquid in a reactor comprising the steps of
moving a drive element in a linear direction within the reactor;
magnetically coupling a follower element to the drive element; and
coupling a foil to the follower element, such that the foil
produces vertical mixing in the liquid in the reactor.
[0079] This invention also provides for a system for the production
of a target molecule or accumulation of biomass comprising a
suspension of cells in a liquid, wherein the cells are capable of
producing a target molecule or accumulating biomass; and a foil
moving by magnetic means in a linear direction in the liquid and
producing vertical mixing of the suspension; wherein production of
the target molecule or accumulation of biomass is greater in the
presence of the moving foil than in the absence of the moving foil;
and wherein the system is a bioreactor.
[0080] This invention also provides for a system for the production
of a target molecule or accumulation of biomass comprising a
suspension of cells in a liquid, wherein the cells are capable of
producing a target molecule or accumulating biomass; and a foil
moving by magnetic means in a linear direction in the liquid and
producing vertical mixing of the suspension; wherein accumulation
of biomass is greater in the presence of the moving foil than in
the absence of the moving foil; and wherein the system is a
bioreactor.
[0081] This invention also provides for a system for achieving
vertical mixing within a fluid in a reactor comprising a foil
moving by magnetic means in a linear direction.
[0082] This invention also provides for a method to achieve
vertical mixing in a fluid in a reactor comprising the steps of
moving a drive element in a linear direction within the reactor;
magnetically coupling a follower element to the drive element; and
coupling a foil to the follower element, such that the foil
produces vertical mixing in the fluid in the reactor.
[0083] This invention also provides for a magnetically coupled
mixing system comprising a mixing vessel; a fluid disposed within
the mixing vessel; a drive conduit; a drive element disposed within
the drive conduit and adapted to move in a longitudinal direction
within the drive conduit; a follower member having a first distal
end and a second distal end, wherein the follower member is
disposed around the perimeter of at least a portion of the drive
conduit and is adapted to move longitudinally along the drive
conduit; a magnetic follower element disposed within the follower
member, wherein the magnetic follower element is adapted to couple
magnetically with the drive element and is proximally disposed
outside the drive conduit; a foil having a surface shaped or
configured to provide lift, wherein the foil is disposed at least
partially in the fluid, and wherein vertical mixing of the fluid is
achieved by linear motion of the foil in the mixing vessel; and a
support member connecting the foil and the follower member.
[0084] This invention also provides for a magnetically coupled
mixing system comprising a mixing vessel; a fluid disposed within
the mixing vessel; a foil having a surface shaped or configured to
provide lift, wherein the foil is disposed at least partially in
the fluid, and wherein vertical mixing of the fluid is achieved by
linear motion of the foil in the mixing vessel; a cable attached to
the foil; and a reversible motor adapted to pull the cable.
[0085] This invention also provides for a magnetically coupled
mixing system comprising a mixing vessel; a liquid disposed within
the mixing vessel; a drive conduit; a drive element disposed within
the drive conduit and adapted to move in a longitudinal direction
within the drive conduit; and a magnetic follower element disposed
around the perimeter of at least a portion of the drive conduit and
adapted to move longitudinally along the drive conduit, wherein the
magnetic follower element is adapted to couple magnetically with
the drive element and is proximally disposed outside the drive
conduit.
[0086] This invention also provides for a system for the production
of a target molecule or accumulation of biomass comprising a
suspension of cells in a liquid, wherein the cells are capable of
producing a target molecule or accumulating biomass; and a foil
moving by magnetic means in a linear direction in the liquid and
producing vertical mixing of the suspension; wherein the total cost
of mixing per unit weight of the target molecule or biomass
produced is lower using the foil to induce vertical mixing than
using a paddlewheel mixer.
BRIEF DESCRIPTION OF DRAWINGS
[0087] These and other features, aspects and advantages of this
invention will become better understood with regard to the
following description, appended claims and accompanying drawings
where:
[0088] FIG. 1 shows a sectional view of a portion of a mixing
system in accordance with certain embodiments of the present
invention;
[0089] FIG. 2 shows an axial view of a portion of a mixing system
in accordance with certain embodiments of the present
invention;
[0090] FIG. 3 shows a perspective view of a portion of a mixing
system in accordance with certain embodiments of the present
invention;
[0091] FIG. 4 shows a perspective view of a portion of a mixing
system in accordance with certain embodiments of the present
invention;
[0092] FIGS. 5A and B show a perspective and sectional view,
respectively, of a follower element in accordance with certain
embodiments of the present invention;
[0093] FIG. 6 shows a sectional view of a portion of a mixing
system in accordance with certain embodiments of the present
invention;
[0094] FIGS. 7A-H show sectional views of exemplary suitable
configurations of drive elements and follower elements;
[0095] FIG. 8 shows a sectional view of a drive element in
accordance with certain embodiments of the present invention;
[0096] FIG. 9 shows a perspective view of a drive element in
accordance with certain embodiments of the present invention;
[0097] FIGS. 10A-C show sectional views of a drive element in
accordance with certain embodiments of the present invention;
[0098] FIGS. 11A-C show sectional and perspective views of a drive
element in accordance with certain embodiments of the present
invention;
[0099] FIGS. 12A-D show sectional and perspective views of a drive
element in accordance with certain embodiments of the present
invention;
[0100] FIG. 13 shows a portion of a mixing system in accordance
with certain embodiments of the present invention;
[0101] FIG. 14 shows a sectional perspective view of a mixing
system and idealized fluid flow in accordance with certain
embodiments of the present invention;
[0102] FIG. 15 shows a sectional perspective view of a mixing
system in accordance with certain embodiments of the present
invention;
[0103] FIG. 16 shows a sectional perspective view of a mixing
system in accordance with certain embodiments of the present
invention;
[0104] FIGS. 17A-C show side and front views of portions of mixing
systems in accordance with certain embodiments of the present
invention;
[0105] FIG. 18 shows a sectional perspective view of an embodiment
of a drive conduit;
[0106] FIGS. 19A and B show sectional views of a drive element in
accordance with certain embodiments of the present invention;
[0107] FIGS. 20A and B show sectional and end views of a drive
element in accordance with certain embodiments of the present
invention;
[0108] FIGS. 21A and B show sectional and end views of a follower
member and bypass conduit in accordance with certain embodiments of
the present invention;
[0109] FIG. 22 shows an embodiment of a hydraulic mixing system in
accordance with certain embodiments of the present invention;
[0110] FIG. 23 shows an embodiment of a pneumatic mixing system in
accordance with certain embodiments of the present invention;
[0111] FIG. 24 shows a planform schematic view of a mixing system
in accordance with certain embodiments of the present
invention;
[0112] FIG. 25 shows a planform schematic view of a mixing system
in accordance with certain embodiments of the present
invention;
[0113] FIG. 26 shows a cable driven mixing system in accordance
with certain embodiments of the present invention;
[0114] FIG. 27 shows a cable driven mixing system in accordance
with certain embodiments of the present invention;
[0115] FIG. 28 shows an embodiment of a foil in accordance with
certain embodiments of the present invention;
[0116] FIG. 29 shows trailing vortices and vertical mixing
generated by horizontal movement of a foil in a mixing system in
accordance with certain embodiments of the present invention;
[0117] FIG. 30 shows a computational simulation of trailing
vortices and vertical mixing generated by horizontal movement of a
foil in a mixing system in accordance with certain embodiments of
the present invention;
[0118] FIG. 31 shows a foil and a flotation member in accordance
with certain embodiments of the present invention;
[0119] FIGS. 32A and B show foils in accordance with certain
embodiments of the present invention;
[0120] FIGS. 33A and B show foils, flotation members, a drive
system and support members in accordance with certain embodiments
of the present invention;
[0121] FIGS. 34A-D show foils in accordance with certain
embodiments of the present invention;
[0122] FIGS. 35A-D show flotation members, support members, foils,
flexible dredging members and brushes in accordance with certain
embodiments of the present invention;
[0123] FIG. 36 shows a foil, flotation member and support members
that are rotatably connected in accordance with certain embodiments
of the present invention;
[0124] FIGS. 37A and B show a portion of a mixing system having a
cambered horizontal support member in accordance with certain
embodiments of the present invention;
[0125] FIG. 38 shows foils, flotation members, support members and
a surface agitating comb in accordance with certain embodiments of
the present invention;
[0126] FIG. 39 shows foils, flotation members and support members
in accordance with certain embodiments of the present
invention;
[0127] FIG. 40 shows a portion of a mixing system and an airfoil in
accordance with certain embodiments of the present invention;
[0128] FIG. 41 shows a comparison of calculated energy requirements
for mixing systems of the present invention with a mixing system
known in the art;
[0129] FIG. 42 shows a comparison of energy consumption in
pneumatic and hydraulic mixing systems in accordance with certain
embodiments of the present invention;
[0130] FIG. 43 shows an exemplary graphical representation of the
dependence of biomass accumulation on mixing type in closed
photobioreactors;
[0131] FIG. 44 shows an exemplary graphical representation of
capital expenditure per hectare for different mixing systems;
[0132] FIG. 45 shows a perspective view of tethered chive elements
in accordance with certain embodiments of the present
invention;
[0133] FIG. 46 shows a side view of an embodiment of the present
invention adapted to produce a breaking wave in shallow liquid;
[0134] FIG. 47 shows a side view of an embodiment of the present
invention producing a breaking wave in shallow liquid; and
[0135] FIG. 48 shows a perspective view of an embodiment of the
present invention producing a breaking wave in shallow liquid.
DETAILED DESCRIPTION OF EMBODIMENTS
Mixer Drive System Design
[0136] FIGS. 1-4 show sectional, perspective and front views of a
tubular follower member 120. In the exemplary embodiment, the
tubular follower member 120 is disposed around a portion of a drive
conduit 100, such that the follower member 120 slides along the
surface of the drive conduit 100 in a longitudinal direction. The
follower member 120 partially encloses a magnetic follower element
110, which is adapted to slide within the follower member 120 along
the surface of the drive conduit 100 in a longitudinal direction.
The distal end portions 192 of the follower member 120 are
partially enclosed, thereby restricting the movement of the
follower element 110 and providing surfaces against which the
follower element 110 can exert force.
[0137] The follower member 120 and drive conduit 100 can be
constructed from, for example, blow-molded or injection-molded
thermoplastic, or any other material that is suitably rigid and
light-weight.
[0138] FIGS. 5A and B show perspective and sectional views of an
exemplary follower element 110, comprising magnets 140, a bushing
150 and an enclosure 160. The annular axial cross-section of the
bushing 150 enables the follower element 110 to slide axially along
the drive conduit 100.
[0139] The bushing 150 can be constructed from, for example,
stainless steel or any other material that is suitably resistant to
wear and has a low coefficient of friction. The enclosure 160 can
be constructed from, for example, polyethylene or any other
material that is suitably durable and has a low coefficient of
friction.
[0140] FIG. 6 illustrates a sectional view of a portion of an
exemplary foil assembly 180 of the present invention. In certain
embodiments, the drive system utilizes magnetic coupling between a
drive magnet 142 or drive ferromagnet 144 contained within the
drive conduit 100 and a follower magnet 140 disposed outside the
drive conduit 100, within the follower member 120, wherein the
magnetic coupling is used in conjunction with a motive force, such
as a pneumatic force or a hydraulic force, to propel the foil
assembly 180 through a mixing vessel.
[0141] In the exemplary embodiment, a tubular follower member 120
is disposed around a portion of a drive conduit 100, such that the
follower member 120 slides along the surface of the drive conduit
100 in a longitudinal direction. The follower member 120 is
connected to a hydrodynamic cambered foil 170 by a support member
130. The distal end portions 192 of the follower member 120 are
partially enclosed.
[0142] In the exemplary embodiment, the follower magnet 140 is a
ring magnet that encompasses the circumference of the exterior
surface of the drive conduit 100. The follower magnet 140 is
adapted to slide within the follower member 120 along the surface
of the drive conduit 100 in a longitudinal direction. The partial
enclosures of the end portions 192 of the follower member 120
restrict the movement of the follower magnet 140 relative to the
follower member 120 and provide surfaces against which the follower
magnet 140 can exert force.
[0143] The drive magnet 142 or drive ferromagnet 144 is disposed
within the drive conduit 100 and is adapted to move longitudinally
within the drive conduit 100 when motive force is applied to the
drive magnet 142 or drive ferromagnet 144.
[0144] In operation, a motive force is applied to the drive magnet
142 or drive ferromagnet 144, which traverses the drive conduit 100
in a longitudinal direction. The follower magnet 140 is
magnetically coupled with the drive magnet 142 or drive ferromagnet
144 and moves in unison with the drive magnet 142 or drive
ferromagnet 144. When the follower magnet 140 comes into contact
with the partial enclosure of either end portion 192 of the
follower member 120, the momentum of the follower magnet 140 is
transferred to the follower member 120, which is propelled by the
follower magnet 140 and slides along the length of the drive
conduit 100. The foil 170 is connected to, and moves with, the
follower member 120, such that the foil 170 traverses through the
mixing vessel and the liquid mixture contained therein in a linear
path.
[0145] The direction in which motive force is applied to the drive
magnet 142 or drive ferromagnet 144 can be reversed, inducing the
drive magnet 142 or drive ferromagnet 144 to move in the opposite
longitudinal direction. The follower magnet 140 is magnetically
coupled with the drive magnet 142 or drive ferromagnet 144 and
correspondingly changes direction of motion with the drive magnet
142 or drive ferromagnet 144. Immediately after the initial change
of direction of motion, the follower magnet 140 disengages from
contact with the partial enclosure of the end portion 192 of the
follower member 120 and traverses the open interior portion of the
follower member 120, during which the follower member 120 will
remain stationary, or will not otherwise undergo motion
attributable to the follower magnet 140. If movement of the
follower magnet 140 is sustained, the follower magnet 140
subsequently comes into contact with the opposite partially
enclosed end portion 192 of the follower member 120 and transfers
momentum to the follower member 120. The follower member 120 and
attached foil 170 consequently undergo a change of direction of
motion.
[0146] In the exemplary embodiment, the distance between the
position of the partially enclosed end portion 192 of the follower
member 120 and the vertical centerline of the support member 130
creates a locus of force, or tow point, between the follower magnet
140 and follower member 120 that is forward of the foil 170
relative to the direction of motion of the foil assembly 180. When
the direction of motion reverses, the tow point becomes the point
of contact between the follower magnet 140 and the opposite end of
the follower member 120, which is likewise forward of the foil 170
relative to the direction of motion of the foil assembly 180. This
configuration enhances guidance and stability of the follower
member 120 and the attached foil 170 by preventing yaw of the foil
assembly 180 while the follower member 120 is in motion in either
direction.
[0147] FIGS. 7A-H illustrate exemplary suitable drive magnets 142
or drive ferromagnets 144 and follower magnets 140. Each drive
magnet 142 or drive ferromagnet 144 is disposed within a drive
conduit 100 and is adapted to move within the drive conduit 100 in
a longitudinal direction in response to a motive force applied to
the drive magnet 142 or drive ferromagnet 144. The drive magnets
142 or drive ferromagnets 144 comprise, for example, one or more
ferromagnetic ball bearings, one or more axially-magnetized
cylindrical magnets, or one or more spherical magnets. One, of
skill in the art will understand that the drive magnet 142 or drive
ferromagnet 144 can be made of any material and have any shape
suitable to promote magnetic coupling with the follower magnet 140
and range of motion within the drive conduit 100. In some
embodiments, the drive magnet 142 or drive ferromagnet 144
comprises steel, a neodymium iron boron magnet or another rare
earth magnet. In some embodiments, the drive magnet 142 or drive
ferromagnet 144 is coated with felt or other materials that are
suitable for suppressing corrosion or wear of the drive magnet 142
or drive ferromagnet 144 and other surfaces that come into contact
with the drive magnet 142 or drive ferromagnet 144.
[0148] In certain embodiments, the drive conduit 100 is a tube
having a circular cross section and is made of low density
polyethylene, high density polyethylene, cross-linked polyethylene,
polyvinyl chloride, copper, steel or any other suitable material.
In some embodiments, the construction of the drive conduit 100
provides positive buoyancy to help maintain the position of the
drive conduit 100 relative to the surface 320 of the liquid
mixture.
[0149] The follower magnet 140 is disposed on or around the
external surface of the drive conduit 100 in a manner that allows
the follower magnet to move along the longitudinal axis of the
drive conduit 100. An example of a suitable follower magnet 140 is
one axially-magnetized ring magnet, wherein the ring magnet
circumferentially encompasses a portion of the drive conduit 100.
Another suitable configuration is a plurality of follower magnets
140 embedded in a follower element 110 in the form of a sliding
sleeve. In some embodiments, the follower member 120 does not fully
encircle the drive conduit 100. In some embodiments, the follower
magnets 140 are disposed on opposite sides of the drive conduit 100
and are equidistant apart relative to the circumference of the
drive conduit 100. One of skill in the art will understand that the
follower magnet 140 may be made of any material and have any shape
suitable to promote magnetic coupling with the drive magnet 142 or
drive ferromagnet 144 and longitudinal range of motion along the
exterior of the drive conduit 100. In some embodiments, the
follower magnet 140 comprises a neodymium iron boron magnet or
another rare earth magnet.
[0150] In the present invention, the gap between the surface of the
drive magnet 142 or drive ferromagnet 144 and the interior surface
of the drive conduit 100 is minimized in order to reduce hydraulic
or pneumatic fluid flow around the drive magnet 142 or drive
ferromagnet 144 and maximize motive force applied to the drive
magnet 142 or drive ferromagnet 144 and the foil 170 for a selected
flow rate of drive fluid. In certain embodiments comprising a
pneumatic fluid used to apply motive force to the drive magnet 142
or drive ferromagnet 144, a low friction seal between the surface
of the drive magnet 142 or drive ferromagnet 144 and the interior
surface of the drive conduit 100 is utilized. The low friction seal
can be created by, for example, dispersing oil along the length of
the interior surface of the drive conduit 100 or by applying a
ferromagnetic fluid to the surface of the drive magnet 142 or drive
ferromagnet 144. In certain embodiments, felt or another suitable
material or coating is adhered or applied to the surface of the
drive magnet 142 or drive ferromagnet 144 to reduce friction
between the drive magnet 142 or drive ferromagnet 144 and the inner
surface of the drive conduit 100.
[0151] FIG. 8 shows a sectional view of a drive element 190
disposed within a drive conduit 100. The drive element comprises a
drive magnet 142 or drive ferromagnet 144 embedded in a plug of
closed cell foam 200, wherein the shape of the plug of closed cell
foam 200 conforms to the inner surface of the drive conduit 100.
The drive element 190 further comprises inserts of open cell foam
210 embedded within the plug of closed cell foam 200. The inserts
of open cell foam 210 are positioned and adapted to expand and
exert outward pressure on the plug of closed cell foam 200 in order
to improve sealing and decrease empty space between the outer
surface of the drive element 190 and the inner surface of the drive
conduit 100.
[0152] FIGS. 9 and 10A-C show another embodiment of a drive element
190, comprising a drive magnet 142 or drive ferromagnet 144, an
o-ring 220, an insert 230, a ring 240 and an end cap 250. The
o-ring 220 may be made of, for example, nitrile rubber. The insert
230 may be made of, for example, nylon 6-6. The ring 240 may be
made of, for example, polytetrafluoroethylene. The end cap 250 may
be made of, for example, nylon 6-6. The outside diameter of the
o-ring 220 is slightly larger than the inside diameter of the drive
conduit 100. The o-ring gland 232 in the insert 230 in which the
o-ring 220 sits is wider than the diameter of the o-ring 220, and
the inside diameter of the o-ring 220 is slightly larger than the
diameter of the o-ring gland 232 in which the o-ring 220 sits, such
that the o-ring 220 sits loosely in the o-ring gland 232. When the
drive element 190 is disposed in the drive conduit 100, the o-ring
220 is squeezed against the inside wall of the drive conduit 100
but is free to move laterally in the o-ring gland 232 (FIG. 10B).
Pneumatic motive force applied to the drive element 190 forces the
o-ring 220 to move in the direction of the motive force until it
contacts the wall of the o-ring gland 232, forming a floating seal
between the o-ring 220, the wall of the o-ring gland 232 and the
inner wall of the drive conduit 100 (FIG. 10C).
[0153] FIGS. 11A-C and 12A-D show a drive element 190 that
incorporates a bypass adapted for use with a mixing system that
incorporates a hydraulic form of motive force. The drive element
190 has an annular shape, and a plunger 260 is disposed at least
partially within the drive element 190 such that the plunger 260
can slide laterally within the drive element 190. When the plunger
260 is positioned such that neither head 262 is in contact with the
drive element 190, fluid can flow around the plunger 260 and
through the annular opening in the drive element 190 (FIG.
11C).
[0154] A cap 270 may be sized and positioned within the drive
conduit 100 such that the cap 270 restricts the lateral motion of
the drive element 190. The cap 270 may be configured so as to hold
the plunger 260 in the open position when the cap contacts the
drive element 190, such that fluid flows through the annular
opening in the drive element 190 and through corresponding channels
272 formed in the cap 270. If any blockage occurs in the drive
system, the configuration shown in FIGS. 11A-C and 12A-D prevents
excess fluid pressure from accumulating.
[0155] As illustrated in FIG. 13, the towing point for rigidly
mounted foils 170 is shifted forward of the flotation members 300,
which act as rudders to provide steering, when the direction of
movement of the foil assembly 180 is reversed. The exemplary
embodiment illustrated in FIG. 13 allows the tow point of the foil
assembly 180 to slide to a stop beyond the flotation members 300 by
allowing the follower element 110 to slide along the drive conduit
100 between end caps 122 that are mounted in fixed positions on the
drive conduit 100. Pads 280 may be attached to the follower element
110 or end caps 122 to reduce the force of impact caused by
acceleration of the foil assembly 180 during a reversal in the
direction of motion. The impact force may also be reduced by
incorporating an alternative attachment between the foil 170,
flotation member 300 or support members 130 and the end caps 122.
The attachment may incorporate flexible tension members 290 made of
for example, 0.125 inch-diameter silicon rubber, under slight
tension. The tow point in this exemplary embodiment also alternates
as a steering element or stabilizing rudder when it is shifted aft
of the foil 170.
[0156] FIGS. 14, 15, 16 and 17A-C illustrate alternative
embodiments of mixing vessels containing liquid algae cultures and
mixing systems in accordance with the present invention. In FIGS.
14 and 15, the drive conduit 100 is disposed in or above the
surface 320 of the algae culture so that it floats in the
photobioreactor 310. The drive conduit 100 may be made from, for
example, high density polyethylene or any other suitable material
that is inexpensive and is durable in saltwater, volatile
compounds, sterilizing agents and moderate heat. In the exemplary
embodiments, the follower element 120 contains follower magnets
140. Magnets of 0.5 inch diameter and 0.5 inch length can achieve a
coupling force of four pounds with minimal lateral force acting on
the drive magnet 142 or drive ferromagnet 144, which would be
exhibited as friction between the drive magnet 142 or drive
ferromagnet 144 and the inner wall surface of the drive conduit
100. The exemplary embodiments also incorporate horizontal support
members 130 positioned above the surface 320 of the algae culture
to reduce hydrodynamic drag, and vertical support members 130
connecting the flotation members 300 to the foils 170 that are
adapted to minimize interference with fluid flow around the foils
170.
[0157] In some embodiments, the drive conduit 100 is located
outside the mixing vessel, and the drive element 190 contained
therein is magnetically coupled to a foil assembly 180 disposed
inside the mixing vessel. As shown in FIG. 16, the drive conduit
100 is disposed underneath the bottom surface of the
photobioreactor 310. The film of the photobioreactor 310 lays over
the drive conduit 100 and a track 352. The weight of the algae
culture provides sufficient hydraulic pressure to fix the position
of the drive conduit 100 and the track 352. A wheeled carrier 350
inside the photobioreactor 310 longitudinally traverses the track
352 and carries a follower magnet 140 that is magnetically coupled
with a drive magnet 142 or drive ferromagnet 144 disposed within
the drive conduit 100. An array of foils 170 are attached to the
wheeled carrier 350. In some embodiments, the drive conduit 100 and
the track 352 are welded or otherwise fixed to the film of the
bioreactor 310.
[0158] Sufficient vertical and horizontal clearances between the
wheeled carrier 350 and the track 352 are maintained to accommodate
for the conformation of the film around the track 352. In the
exemplary embodiment, a set of 3, 0.5 inch long follower magnets
140 provides 2 pounds of coupling force in the axial direction,
which is sufficient to maintain coupling during sudden
decelerations of the drive element 190 when stopped or started at
the ends of the photobioreactor 310. This arrangement of follower
magnets 140 also provides a torque and downward attraction force of
approximately 4 pounds that prevents the wheeled carrier 350 from
separating from the track 352.
[0159] The embodiments illustrated in FIGS. 14 and 15 eliminate the
need to lay the film of the photobioreactor 310 carefully over the
drive conduit 100 in the embodiment of FIG. 16 in order to avoid
wrinkles that might structurally compromise the film of the
photobioreactor 310 and the need to have a flat track 352
underneath the photobioreactor 310. In the embodiments of FIGS. 14
and 15, there is no wheeled carrier 350 rolling on the floor of the
photobioreactor 310, so the thin film of the photobioreactor 310 is
not exposed to continual mechanical stress on its surface, which
eventually may lead to failure of the film.
[0160] Photobioreactors 310 also may deflect horizontally over
lengths of, for example, 50 feet. With the exemplary embodiment
illustrated in FIG. 16, deflection of more than a few inches may
cause a foil 170 to impact the film on the side of a
photobioreactor 310 and potentially tear the film, if no guard
mechanism is in place, and if the drive conduit 100 and the track
350 are not fixed to an inside surface of the bioreactor 310. In
the embodiment illustrated in FIG. 15, if the photobioreactor 310
curves excessively, the wall of the photobioreactor 310 would push
the tracking member 340 above the surface 320 of the algae culture
and displace the foil assembly 180 laterally, since the drive
conduit 100 would offer minimal bending resistance. The tracking
members 340 thereby guard against accidental contact between the
foils 170 and the film on the sides of the photobioreactor 310.
[0161] FIGS. 17A-C illustrate exemplary embodiments having a drive
conduit 100 positioned underneath, or on any side of, a
photobioreactor 310. One or more drive magnets 142 or drive
ferromagnets 144 are contained in the drive conduit 100. One or
more follower magnets 140 are attached to a flotation member 300 by
support members 130 and are magnetically coupled with the drive
magnets 142 or drive ferromagnets 144.
Motive Force System
[0162] The present invention utilizes a pump 420 to provide motive
force to the drive elements 190 in an array of mixing vessels, such
as photobioreactors 310. In some embodiments, one pump 420 can
drive mixing in multiple vessels.
[0163] The pump 420 moves drive fluid through the drive conduit
100. The drive fluid may be, for example, air, water, mineral oil
or polyethylene glycol, and the drive fluid may be suffused with
agents that inhibit corrosion. In some embodiments, a drive fluid
of low density is selected for use in the drive conduit 100 to
promote positive buoyancy of the drive conduit 100, in particular
if the drive conduit 100 is constructed of materials having high
density, such as steel or iron.
[0164] FIGS. 22 and 24 show exemplary embodiments in Which a drive
conduit 100 served by a single pump 420 and a switching valve 430
is routed through multiple photobioreactors 310 in a serpentine
configuration. One or more drive elements 190, follower elements
110 and foil assemblies 180 can be disposed inside each
photobioreactor 310. In the exemplary embodiment, the pump 420 and
switching valve 430 are located outside the photobioreactors 310
and the drive system is closed.
[0165] FIG. 22 illustrates an exemplary hydraulic mixing system in
which one pump 420 drives mixing in four rows of at least 70
photobioreactors 310 per row. Each row of photobioreactors 310 is
partitioned into groups of 12 photobioreactors 310 through which a
single drive conduit 100 of 0.5 inch diameter passes. Flow is
distributed to the rows of photobioreactors 310 via the drive
conduits 100 using a two inch diameter pipe 410. A pump 420 pushes
water through the pipes 410 and drive conduits 100 to drive one or
more drive elements 190 in the drive conduits 100 in each
photobioreactor 310. The drive elements 190 are magnetically
coupled to foil assemblies 180, which comprise follower elements
110, follower members 120, foils 170 and connecting support members
130.
[0166] The exemplary hydraulic mixing system of FIG. 22 comprises a
pump 420 rated at one horsepower, 48 gallons per minute and 200
foot head (approximately 100 pounds per square inch). The exemplary
hydraulic mixing system further comprises a four-way air piloted
valve 430 adapted to switch the direction of hydraulic flow, a flow
control valve 440, a flow meter 450 and an air bleed tank 460. The
hydraulic pressure in the pipe 410 is approximately 30 pounds per
square inch.
[0167] The exemplary hydraulic mixing system of FIG. 22 moves the
drive elements 190 and foil assemblies 180 at a constant speed for
a preselected length of time. In some embodiments, a length of time
greater than 30 seconds is expected to be sufficient for the drive
elements 190 and foil assemblies 180 to traverse the full length of
a photobioreactor 310. Mechanical stops (not shown) at the ends of
each photobioreactor 310 restrict the movement of the drive
elements 190 and foil assemblies 180 while hydraulic flow continues
for a short period of time to enable the drive elements 190 and
foil assemblies 180 to reach the end of each photobioreactor 310.
In some embodiments, the short period of time is five to ten
seconds.
[0168] In the exemplary hydraulic mixing system, the four-way air
piloted valve 430 subsequently is activated to reverse the
hydraulic flow throughout the drive conduit 100. The activation of
this valve 430 preferably occurs slowly enough to avoid dislodging
the magnetic coupling between the drive elements 190 and follower
elements 110. In some embodiments, the length of time for
activation of the valve 430 is greater than 100 milliseconds.
[0169] In alternative embodiments, a reversible positive
displacement pump 420 with a variable frequency drive is used and
the four-way air piloted valve 430 is omitted. The hydraulic flow
through the system is controlled to a specified rate.
[0170] Energy usage can be reduced by reducing the cross-sectional
area of the drive conduit 100 or changing the drive fluid to air.
Reducing the cross-sectional area of the drive conduit 100 requires
closer tolerances between the drive element 190 and follower
element 110 so that smaller magnets can be used. Changing the drive
fluid to air requires using a sealing fluid around the drive magnet
142 or drive ferromagnet 144 to avoid wasting energy.
[0171] FIG. 23 illustrates an exemplary pneumatic mixing system in
which one pump 420 drives mixing in 240 photobioreactors 310
connected in parallel. Drive conduits 100 of 0.5 inch diameter pass
through each row of photobioreactors 310. Flow is distributed to
the rows of photobioreactors 310 via the drive conduits 100 using a
two inch diameter pipe 410. A pump 420 pushes air through the pipe
410 and drive conduits 100 to drive one or more drive elements 190
in the drive conduits 100 in each photobioreactor 310. The drive
elements 190 are magnetically coupled to foil assemblies 180, which
comprise follower elements 110, follower members 120, foils 170 and
connecting support members 130.
[0172] In some embodiments, a system of the present invention
incorporates multiple drive elements 190 and foil assemblies 180 in
one mixing vessel. The foil assemblies 180 are disposed on the
drive conduit 100 at selected distance intervals and are driven by
the same motive system. The foil assemblies 180 may be configured
and spaced to provide mixing over the full length of the mixing
vessel, wherein each foil assembly 180 provides mixing for a
selected portion of the mixing vessel. One of ordinary skill will
appreciate that the lowest energy consumption required to achieve a
desired degree of vertical mixing in a photobioreactor 310 can be
determined by varying the number and configuration of foil
assemblies 180 that are used in the photobioreactor 310.
[0173] FIG. 24 illustrates an embodiment in which multiple
photobioreactors 310 are serviced by one pump 420 and one drive
conduit 100. The photobioreactors 310 are connected in series. In
this embodiment, multiple foil assemblies 180 are disposed inside
each photobioreactor 310 and are configured to traverse the length
of the bioreactor in opposite directions.
[0174] FIG. 25 illustrates an embodiment in which multiple
photobioreactors 310 are serviced by one pump 420 and multiple
drive conduits 100 that are fed by a header pipe 410. Pairs of
photobioreactors 310 are connected in parallel, and the
photobioreactors 310 in each pair are connected in series. In this
embodiment, multiple foil assemblies 180 are disposed inside each
photobioreactor 310 and are configured to traverse the length of
the bioreactor in the same direction.
[0175] In embodiments of the present invention that utilize
pneumatic force to move foil assemblies 180, multiple foil
assemblies 180 may be disposed inside each photobioreactor 310,
while multiple photobioreactors will be connected in parallel,
instead of in series. As shown in FIG. 45, multiple drive elements
190 are tethered by a connector 252, which maybe any flexible or
rigid elongate member made of, for example, plastic, nylon or
elastomer, that physically connects the drive elements 190.
Unidirectional floating seal grooves 222 are formed in the drive
elements 190 positioned at the distal end of each chain of drive
elements 190. When pneumatic motive force is applied, the
unidirectional floating seal groove 222 formed in the drive element
190 in the upstream position allows gas to slip underneath the
o-ring 220 and prevent the formation of a seal. A seal is instead
formed by the o-ring 220 present in the downsteam drive element
190. The downstream drive element 190 moves in the direction of the
pneumatic motive force and pulls the upstream drive element 190 in
the same direction. If the chain of drive elements 190 includes
three or more drive elements 190, then only the drive element 190
at the upstream end of the chain and the drive element 190 at the
downstream end of the chain have o-rungs 220, so that no seals are
formed by the drive elements 190 positioned in the interior of the
chain.
[0176] In embodiments of the present invention that utilize
hydraulic force to move foil assemblies 180, a disadvantage of
servicing multiple vessels in series using one pump 420 is that an
increased pressure drop caused by a decreased rate of mixing in one
vessel, such as due to a stopped drive element 190, affects mixing
speed in all other vessels in the array. In some embodiments, a
system of the present invention comprises a positive displacement
pump 420, such as a vane pump, for the purpose of rendering flow
rate through the pump 420 independent of pressure variations and
providing consistent mixing speed in the array of vessels.
[0177] Operating pressure in a system of the present invention
utilizing hydraulic force is approximately 32 pounds per square
inch, while maximum pressure induced by stoppages of the drive
elements 190 in twelve mixing vessels may be as high as 360 pounds
per square inch. A large pump motor 420 may generate sufficient
torque to maintain constant mixing speed under temporary large
pressure drops in the mixing system but will not operate
efficiently under smaller pressure drops that are typical during
normal operation of the mixing system, when no drive elements 190
are stopped. With reference to FIG. 18, in some embodiments, the
conduit used in a system of the present invention allows flow past
a stopped drive element 190 and reduces the pressure drop, which
reduces the size of the pump motor 420 needed for the system, by
incorporating a small channel 360 between the surface of the drive
element 190 and the inner surface of the drive conduit 100. The
channel 360 is sufficiently small to maintain motive force in the
form of pressure from a drive fluid applied to the drive element
190 under normal operation, and the change in outside diameter of
the drive conduit 100 is sufficiently small to avoid interference
with a follower element 110. If channels 360 are formed in the
inner surface of the drive conduit 100 and the outside diameter of
the drive conduit 100 is unchanged, a higher schedule drive conduit
100 must be selected to compensate for the loss of pressure
capability in the drive conduit 100.
[0178] As illustrated in FIGS. 19A and B and FIGS. 20A and B, in
some embodiments, the drive element 190 incorporates a poppet valve
370. If a misaligned or otherwise impeded drive element 190 creates
a blockage in the drive conduit 100, the poppet valve 370 is
adapted to allow drive fluid to flow through the drive element 190
and provides an alternative means of releasing pressure. In some
embodiments, the internal surface of the drive conduit 100
incorporates drive stops 380 that restrict the motion of the drive
element 190 in a longitudinal direction within the drive conduit
100. In some embodiments, a valve opening pin 390 is adapted to
actuate the poppet valve 370.
[0179] As illustrated in FIGS. 21A and B, some embodiments comprise
a bypass conduit 400 that is formed in the drive conduit 100. The
bypass conduit 400 is adapted to relieve fluid pressure by
diverting the flow of drive fluid in the event that a blockage
occurs in the drive conduit 100 between the inlet and outlet of the
bypass conduit 400. In some embodiments, the follower member 120 is
shaped to accommodate the placement of the bypass conduit 112.
[0180] As illustrated in FIGS. 26, 27 and 28, in some embodiments
the motive system is a cable connected system. A cable 470 attaches
to a foil 170 and/or to flotation members 300 in a foil assembly
180. The cable 470 is pulled by a motor 472 that is configured to
pull the cable 470 in more than one direction. The foil 170 is
moved by actuating the motor 472 to pull the cable 470, and the
cable 470 is wound on a spool 474 at either end of the mixing
vessel depending on the direction of travel.
Foil Design
[0181] A system in accordance with certain embodiments of the
present invention comprises a surface, or foil 170, that is shaped
to provide hydrodynamic lift, wherein the foil 170 can be moved
through an aqueous culture of algae to efficiently generate a
vertical movement of algae. FIGS. 14, 15, 16 and 28 illustrate
exemplary foils 170 used in a mixing system of the present
invention. It is desirable to induce flow with the lowest velocity
needed to provide satisfactory mixing while minimizing energy
consumption. Net vertical flow of the aqueous culture in the mixing
vessel is zero. In the lowest energy case, upward and downward
velocities are equal and act over equal areas.
[0182] FIG. 14 illustrates idealized vertical fluid motion in
trailing vortices 330 generated by foils 170 in a photobioreactor
310. The trailing vortices 330 remain stationary in the middle of
the photobioreactor 310 due to net cancelation of induced velocity
from image vortices (not shown) that are created at the top and
bottom of the aqueous culture. Generating trailing vortices 330 in
the center of the photobioreactor 310 may maximize the mixing
length, or vertical particle displacement, over which the vortices
330 can transport flow. Trailing vortices 330 in the center of the
photobioreactor 310 form a stable arrangement and tend not to
migrate in location. In contrast, pairs of counterrotating vortices
330 move vertically in unbounded flows or move laterally near a
horizontal surface.
[0183] In the exemplary embodiment, foils 170 distributed along the
span of a photobioreactor 310 provide a regular arrangement of
mixing vortices 330 that minimize the presence of dead zones with
no mixing. Such mixing is sufficient to sustain algae cultures over
extended periods of time. Aqueous algae cultures can die from
anoxia due to reduced gas exchange and sedimentation of the algae
if mixing is stopped for a period of several hours.
[0184] A foil assembly 180 generally may be moved at a higher speed
with downwardly concave foils 170, instead of upwardly concave
foils 170. However, downwardly concave foils 170 produce trailing
vortices 330 that induce sedimentation of algae in regions away
from the foils 170, while upwardly concave foils 170 produce
trailing vortices 330 that rotate in the opposite direction and
induce sedimentation of algae in the region underneath the foil
170. Additional mixing and resuspension of settled algae may be
facilitated to a greater extent by a pattern of sedimentation
underneath the foil 170 rather than sedimentation away from the
edges of the foil 170.
[0185] The migratory speed of trailing vortices 330 can be
predicted by determining the velocity a vortex 330 will induce on
its neighbor, according to potential flow theory and in situations
with surfaces, including the effect of image vortices. For a vortex
330 near a single horizontal surface, such as the bottom of the
photobioreactor 310, the image vortex (not shown), which is a
vortex 330 with opposite rotation placed equidistant but on the
opposite side of the horizontal surface, will induce the actual
vortex 330 to move laterally with a certain speed. When the actual
vortex 330 is bounded on the opposite side by a second horizontal
surface, which is the top surface 320 of the liquid mixture in
exemplary embodiments of the present invention, the image vortex
from this surface 320 will induce a directed motion countering the
induced motion from the lower surface. If the vortex 330 is
centered between these two surfaces, then the induced effect
cancels completely and the vortex 330 remains stationary. The
stability of the vortex 330 position allows for subsequent passes
of a foil 170 to reinforce the strength of the vortex 330.
[0186] The trailing vortices 330 decay slowly and are continually
reinforced as the foils 170 longitudinally traverse the
photobioreactor 310 in either direction, allowing the foils 170 to
effectively mix a large area of aqueous culture in the vessel
compared to the planform area of each foil 170. This efficiency
reduces the amount of equipment needed for mixing and capital
costs.
[0187] The preferred placement of the foil 170 is middepth in the
aqueous culture, with the span of the foil 170, and the lateral
spacing between foils 170, equal to the depth of the culture. This
placement produces a stable configuration of trailing vortices
330.
[0188] The trailing vortices 330 are strengthened by increasing the
lift generated by the foil 170. Lift is controlled by the planform
area, angle of attack, camber and speed of the foil 170. The
trailing vortices 330 are also reinforced by multiple passages of
each foil 170, which can be increased by increasing foil 170 speed
for a photobioreactor 310 of a fixed length or by employing
multiple foil assemblies 180 along the length of one
photobioreactor 310. Foils 170 can be spaced equidistant along the
transverse axis of the photobioreactor 310 to reduce the tine over
which a trailing vortex 310 will decay before reinforcement and
thereby achieve desired recirculation with uniformity and low power
requirements.
[0189] FIG. 29 illustrates measurements of a trailing vortex 330
pair generated by a single foil 170 in an algae culture having a
depth of eight inches. The particle traces shown cover the depth of
a photobioreactor 310, eight inches, and show the flow pattern of
the particles after the passage of one foil 170. Movement of the
foil 170 (not shown) is in a plane perpendicular to the plane of
the image. The trailing vortices 330 were generated by the distal
ends of a foil 170 designed to produce similar recirculation times
in the aqueous culture when the foil 170 longitudinally traverses
the photobioreactor 310 at mid-depth in the algae culture. The foil
170 shape is symmetric and cambered to generate a vortex 330 system
with rotations that are independent of the direction of traverse.
The trailing vortices 330 were measured through laser diagnostics.
The particles are neutrally buoyant and were illuminated with a
laser light sheet. Four consecutive exposures are superimposed in
this image to show the movement of the particles over intervals of
66.7 milliseconds. Total plotted time for each particle is
therefore 4.times.66.7 milliseconds=0.26 seconds.
[0190] FIG. 30 illustrates a computational simulation of trailing
vortices 330 generated by a foil 170 passing through an algae
culture with a depth of eight inches. This image provides the same
view of particle traces shown in FIG. 29. Each trace in FIG. 30
represents a total travel time of 8.21 seconds per particle.
Particles in this simulation are nonuniformly distributed at an
initial time measurement, with most of the particles being located
at mid-depth in the algae culture in a photobioreactor 310 at
time=0 seconds. For a given position along the length of the algae
culture in the photobioreactor 310, the vortices 300 decay over
time and must be reinforced or regenerated by the periodic passage
of the foil 170. The maximum time to reinforce trailing vortices
330 is roughly 30 seconds between passages for a foil 170
travelling at 0.5 meters per second.
[0191] In some embodiments, the foil assembly 180 comprises a
symmetric, cambered foil 170. One of skill in the art will
recognize that other configurations are also suitable to produce
effective mixing and generate trailing vortices 330. Trailing
vortices 330 can be generated by hydrodynamic drag on flat foils
170 that have no camber and are held at a constant angle of attack
as the foils 170 traverse a photobioreactor 310. FIG. 31
illustrates a foil 170 with a quadrangular planform shape that can
produce hydrodynamic lift.
[0192] FIGS. 32A and B illustrate exemplary foils 170 that are
mounted on axles 510 connected to the foil assembly 180 and are
adapted to swivel or pivot to an inclined orientation. The angle of
attack, or pitch, of the foil 170 may be maintained by differential
weighting along the chord of the foil 170. Triangular and
quadrangular planform shapes are suitable for foils 170 of these
embodiments. Differential weighting between the sum of the
hydrodynamic lift and the weight of the foil 170 may be adjusted by
incorporating a weight 480, or an interstitial space 490, embedded
in the foil 170. The angle of attack and the amount of circulation
generated may thereby be varied and adjusted.
[0193] With respect to foils 170 that incorporate embedded weights
480, care must be taken to ensure that the trailing edges of foils
170 with long chords do not scrape the bottom of the mixing vessel
during a reversal in travel direction of the foil 170, as the foil
170 swings underneath the axle 510. With a buoyant trailing edge
incorporating an interstitial space 490, this is avoided since the
trailing edge of the foil 170 would swing over the axle 510.
[0194] With reference to FIGS. 32A and B, foils 170 that swivel
preferably incorporate a rudder 500 to prevent yaw while avoiding
the need to pull the foil 170 from a shifting forward location.
[0195] FIGS. 33A and B illustrate an embodiment comprising foils
170 positioned in a vertical orientation. Trailing vortices 330 can
be generated by vertical foils 170 that are oriented at slight
angles to the direction of the foil 170 movement. In some
embodiments, the vertical foils 170 have a triangular planform
shape.
[0196] In some embodiments, foils 170 are made from molded plastic,
fiberglass, sintered nylon, glass-reinforced plastic, or any other
material that is suitable to provide rigidity, durability, and
positive or neutral buoyancy.
[0197] FIGS. 34A-D illustrate an embodiment comprising
vertically-oriented foils 170 that are designed increase the extent
of mixing directly beneath a horizontally-oriented, upwardly
concave foil 170 in region where settling may occur. The two
vertically-oriented foils 170 are set in opposition to yield net
zero horizontal lift. The two vertically-oriented foils 170 create
a set of trailing vortices 330 that impinge on the liquid culture
directly beneath the vertically-oriented foils 170 and create a
high shear zone that promotes mixing and reduces sedimentation
beneath the foils 170.
[0198] FIGS. 35A and B illustrate the use of a chain 520 or other
suitable flexible dredging member to reduce sedimentation in the
aqueous culture. In this exemplary embodiment, a chain 520 is
attached to, and suspended from, the foil 170 and drags through the
space beneath the foil 170 where sediment accumulates.
[0199] If sedimentation is a more severe problem than the need for
vertical mixing, foils 170 can be omitted from the foil assembly
180 and a uniform brush 530 can be attached to the foil assembly
180 instead, as illustrated in FIGS. 35C and D. The density of the
brush 530 bristles must be sparse enough to avoid excessive
hydrodynamic drag forces. Sufficient downward force must be applied
to the brush 530 by, for example, utilizing the weight of the
support to scrape settled algae from the bottom of the mixing
vessel.
[0200] As illustrated in FIG. 36, a foil 170 can be rotatably
attached to a vertical support member 130 at a pivot point 540.
When the foil 170 is traversing the photobioreactor 310, the
opposing force exerted by the algae culture causes the foil 170 to
rotate away from the direction of travel, thereby tilting the foil
170 and holding the foil 170 at an angle of attack. This effect
creates hydrodynamic drag and turbulence at the trailing edge of
the foil 170, which enhances mixing. When the foil 170 reverses
direction, the foil 170 swings in the opposite direction in a
pendulum fashion and correspondingly creates turbulence and mixing
in the same manner.
Supporting Structure
[0201] There are several ways to maintain the foil 170 at a
constant pitch and with minimal roll. In an exemplary embodiment
illustrated in FIG. 28, the foil 170 is attached to tensioned
guidelines 470. Thin rods are attached to the leading and trailing
edges of the foil 170.
[0202] In embodiments shown in FIG. 31 and FIGS. 35-D, the foil 170
is attached to a flotation member 300, such as, for example, a
pontoon, which reduces frictional losses. The wave pattern
generated by the leading end of the flotation member 300
additionally increases mixing at the surface 320 of the algae
culture to enhance gas exchange, light penetration and
photosynthesis. Counter-rotating longitudinal vortices 330 near the
surface 320 are also generated in the wake of the flotation member
300 and can be enhanced by specific hull designs. The width and
lateral weight distribution of the flotation member 300 control
roll, which is essential to maintain the foil 170 at a constant
depth, while the length of flotation member 300 controls pitching
and thus the angle of attack of the foil 170.
[0203] As illustrated in FIGS. 37A and B, agitation of the surface
320 can be enhanced by inducing a small hydraulic jump 550 by
towing a foil 170 or a cambered horizontal support member 130 just
below the surface 320 of the algae culture.
[0204] As illustrated in FIG. 38, in some embodiments, a surface
agitating comb 560 or other suitable ancillary structure is
attached to a horizontal support member 130 to agitate the surface
320 of the algae culture and increase gas transfer rates between
vapor and liquid phases contained in a photobioreactor 310.
[0205] As illustrated in FIG. 39, in some embodiments, one or more
vertical support members 130 connecting a foil 170 to a flotation
member 300 serve as rudders to prevent yaw. To be effective as
steering devices, the foil 170 needs to pulled from a point forward
of the center of pressure for the vertical support members 130
acting as rudders. A larger distance between the tow point and the
center of pressure for the vertical support members 130 requires a
stronger restoring force to align the foil 170 and counter any
imbalance caused by imperfect foil 170 manufacturing, fouling from
algae growth on the foil 170 or friction from contacting a surface
of the photobioreactor 310. The vertical support members 130
additionally agitate the surface 320 of the algae culture.
[0206] In some embodiments, flotation members 300 and support
members 130 are made from molded plastic, fiberglass, sintered
nylon, glass-reinforced plastic, or any other material that is
suitable to provide rigidity, durability, and positive or neutral
buoyancy.
[0207] As illustrated in FIG. 40, in some embodiments, an airfoil
580 is positioned above the surface 320 of liquid in a.
photobioreactor 310. This configuration advantageously induces
circulation of the vapor phase in the photobioreactor 310, which
helps to increase the efficiency of a solar still by enhancing
vapor transport from the surface 320 of the water to the walls of
the photobioreactor 310.
[0208] In some embodiments, the longitudinal axis of the
photobioreactor 310 or other mixing vessel is vertical. The drive
conduit 100 likewise is vertically oriented, and the foil assembly
180 moves in a vertical direction along the drive conduit 100 and
is fully submersed in the contents of the mixing vessel over at
least a portion of the mixing vessel.
[0209] In some embodiments, the foil assembly 180 is used to skim
the surface 320 of an open pond. The drive conduit 100 is
positioned at or near the surface 320 of the pond, and the position
and angle of attack of the foil 170 is adapted to maintain the foil
170 at or immediately beneath the pond surface 320.
[0210] In an exemplary embodiment, a gas sparging hose is attached
to a foil 170. Gas may be pumped through the hose and bubbled into
the algae culture, with the outlet of the hose located, for
example, underneath the foil 170, near an edge of the foil 170 or
immediately underneath the surface 320 of the algae culture. The
hose is adapted to move along the length of the photobioreactor 310
with the foil 170. Movement of the foil 170 creates shear in the
algae culture near the lateral edges of the foil 170, which shears
and reduces the size of the gas bubbles. Additionally, the gas
bubbles are entrained in the trailing vortices 330, which increases
the residence time of the gas bubbles in the algae culture and
improves efficiency of mass transfer between the liquid phase of
the algae culture and the gas phase present in the photobioreactor
310.
[0211] The hose supplying the gas may be made of any suitable
materials that are impermeable to the gas, that provide buoyance
for the hose to float on the surface 320 of the algae culture and
that are sufficiently pliable to enable the hose to fold or coil on
the surface 320 of the algae culture. In some embodiments, a cage
additionally is attached to the foil assembly 180 to capture slack
in the hose and prevent tangling.
[0212] In an exemplary embodiment, one or more Venturi tubes are
incorporated in support members 130, with one opening of each
Venturi tube disposed above the surface 320 of the algae culture
and the opposite opening of each Venturi tube disposed below the
surface 320 of the algae culture. The diameter of each Venturi tube
may be, for example, approximately 1 to 2 millimeters.
[0213] As the foil 170 moves through the algae culture, a pressure
gradient develops across the length of the Venturi tube. The
pressure gradient pulls gas into the algae culture from above the
surface 320 of the algae culture, creating small bubbles that are
expelled into the algae culture. In some embodiments, Venturi tubes
in the support members 130 are formed symmetrically to provide
equivalent gas bubbling in each direction of longitudinal motion of
the foil 170.
[0214] In an exemplary embodiment using pneumatic motive force to
drive the drive element 190, the foils 170, support members 130 and
other parts of the foil assembly 180 are omitted and only a
follower element 110, which is magnetically coupled to the drive
element 190, moves along the length of the drive conduit 100. When
driven at high speeds, a cylindrical follower element 110 creates
an air cavity in the shape of a bell behind the follower element
110 as it moves through the algae culture. The plunging jet of the
liquid bell causes the formation of small bubbles in the algae
culture, which increases mixing and mass transfer from the algae
culture.
[0215] In some embodiments in which the foil assembly 180 is
omitted, a gas sparging hose is attached to the follower element
110. The hose introduces gas bubbles into the algae culture. The
end of the hose that is not attached to the follower element 110
may be connected to the photobioreactor 310 along the side in the
center of the photobioreactor 310.
Mixing Operations
[0216] A mixing system in accordance with some embodiments of the
present invention is capable of generating vertical mixing that is
essential to the cultivation of algae in a photobioreactor 310
while minimizing capital investment and energy usage. In shallow
depths with vertical recirculation, the necessary minimum vertical
velocity needed to maintain a culture comprising certain strains of
algae and to prevent visible sedimentation is approximately 20 to
30 seconds for turnover of algae in a circular cross section of six
to eight inches contained in a culture having a depth of eight
inches. For circulation rates that exceed this threshold,
production in these systems increases only slightly, while energy
consumption increases significantly.
[0217] FIG. 41 illustrates calculated energy requirements for
mixing in a raceway pond system known in the art, as compared with
energy requirements for certain embodiments of the present
invention, to account for sliding friction, hydraulic and pneumatic
losses and motor efficiency. The system of the present invention
advantageously consumes energy at a lower rate while providing
sufficient mixing of an aqueous algae culture. The smaller and
larger columns for the raceway system represent, respectively,
energy consumption required for 40 second and 20 second turnover
times. This range of circulation tunes corresponds to the initial
circulation time (20 seconds) after a foil assembly 180 passes
through a static point in the mixing vessel and the decayed
circulation time (40 seconds) immediately before the foil assembly
180 subsequently passes through the same point again, when the foil
assembly 180 is towed at 0.5 meters per second.
[0218] A major distinction between rotary impeller motion known in
the art and the linear foil 170 motion comprised by the present
invention is the functionality and performance of the type of
motion. Rotary impeller, or paddle wheel, motion is generally
utilized to generate a directed flow, typically along the length or
circumference of the vessel. Fluctuations in motion that are
transverse to the selected direction of flow are chaotic and occur
due to turbulence that is generated in the boundary layer of the
vessel. Components of turbulence that are not directed vertically
and do not contribute to vertical transport, which is important in
the embodiment of a photobioreactor 310 where light is received
from overhead, are nonetheless generated. The chaotic vertical
motion and extra fluctuation components thus make this rotary
motion energetically inefficient for bulk vertical transport.
[0219] The efficiency of vertical transport may be increased by
placing foils 170 in a stream (as taught by Laws et al.) to induce
vertical recirculation. However, the majority of flow kinetic
energy is still dissipated through overcoming boundary friction in
sustaining the relative motion required.
[0220] In accordance with embodiments of the present invention, by
moving the foil 170 in a linear path through an algae culture
contained in a photobioreactor 310 or other mixing vessel, the
problem of dissipation of flow kinetic energy is overcome. The
decay of recirculation, which is created by passing a foil 170
through the algae culture in a linear path to generate trailing
vortices 330, is also ameliorated by repeatedly passing the foil
170 through the same path to continually reinforce the vortices
330.
[0221] Vertical mixing systems known in the art may be compact, at
the expense of elevated energy consumption. In accordance with
embodiments of the present invention, components of the vertical
mixing system may be manufactured from lightweight materials, such
as plastics, which can help minimize capital expenditures and
energy consumption for the system.
[0222] FIG. 42 details energy in pneumatic and hydraulic mixing
systems of the present invention. In the exemplary embodiment, the
pneumatic mixing system uses air and the hydraulic mixing system
uses water. Consumption data attributable to drive element
friction/fluid leakage and to drive conduit/pipe friction are
specific to the motive force used in each mixing system. Drive
element friction/fluid leakage indicates energy loss due to
friction between the outer surface of the drive element 190 and the
inner surface of the drive conduit 100, in conjunction with head
loss due to fluid leaking past the drive element 190 inside the
drive conduit 100. Drive conduit/pipe friction indicates energy
loss due to friction as the drive fluid flows through the pipes 410
and drive conduit 100 of the mixing system. FIG. 42 illustrates
that combined drive element friction/fluid leakage and drive
conduit/pipe friction are substantially lower for the pneumatic
mixing system than for the hydraulic mixing system.
[0223] In FIG. 42, foil assembly indicates energy consumption
attributable to hydrodynamic drag on the foil assembly 180 moving
through the algae culture, which is independent of the drive system
used. Comparison of total energy consumption and energy losses
attributable to each category shows that a substantially higher
proportion of energy is translated directly to moving the foil
assembly 180 using the pneumatic mixing system than using the
hydraulic mixing system.
[0224] In some embodiments, a mixing system of the present
invention is used to agitate algae culture in a photobioreactor 310
intermittently. According to the present invention, intermittent
operation of the mixing system provides sufficient vertical mixing
while economizing energy consumption.
[0225] In exemplary embodiments, mixing systems of the present
invention are used to agitate only portions of algae culture
contained in a photobioreactor 310. In some embodiments,
horizontally oriented foils 170 generate trailing vortices 330 that
agitate the algae culture from the surface 320 to a depth that is
less than the total depth of the algae culture. The span of each
foil 170 may be less than the total depth of the algae culture, so
that the foil 170 generates trailing vortices 330 having diameters
less than the depth of the algae culture 113. Lateral spacing
between foils 170 may be greater than the span of each foil
170.
[0226] In some embodiments, vertically oriented foils 170, each
having a span less than the depth of the algae culture, agitate the
algae culture from the surface 320 to a depth that is less than the
total depth of the algae culture.
[0227] In some embodiments, the foil assembly 180 traverses only a
portion of the length of the photobioreactor 310, agitating the
algae culture contained in that portion while leaving the algae
culture in the remaining portion of the photobioreactor 310
unmixed.
EXAMPLE 1
[0228] Both hydraulic and pneumatic drive fluids have been used to
propel a magnetically coupled foil assembly 180 through a
photobioreactor 310 at 0.5 meters per second. In a preferred
embodiment, the foil assembly 180 traverses the 50 foot length of a
commercial scale photobioreactor 310 at 0.5 meters per second for a
defined time interval (usually 30 seconds) before the foil assembly
180 reverses direction of motion. The minimum steady state power
requirement for motion of a foil assembly 180 according to these
specifications has been determined by measuring fluid pressures and
flow rates while the foil assembly 180 moves at a constant speed.
This determination neglects any additional energy consumption or
efficiency that occurs during the few seconds when the foil
assembly 180 is moving at less than 0.5 meters per second while
reversing direction of motion.
[0229] Using hydraulic drive fluid and a drive ferromagnet 144,
measurements indicate a pressure drop of 8 to 9 pounds per square
inch per photobioreactor 310 at a fluid flow rate of 2 gallons per
minute. Without accounting for pump 420 and drive efficiency, the
minimum power requirement using water as a drive fluid is
determined by Equation 1:
Power.sub.min=.DELTA.Pressure.times.Flowrate=7.8 Watts per
photobioreactor 310
[0230] which is equivalent to 0.34 Watts/m.sup.2 for a 23 m.sup.2
photobioreactor 310.
[0231] Power requirements have not varied in photobioreactors 310
containing either freshwater, seawater or algae culture.
Accordingly, power consumption by mixing systems of the present
invention used in liquid not containing algae culture is
substantially equivalent to power consumption by mixing systems of
the present invention used in liquid containing algae culture.
According to the present invention, drag on the foil assembly 180
is mostly due to inertial forces, rather than viscous forces, and
the densities of all fluids tested in the photobioreactor 310 are
roughly equivalent.
[0232] A preponderance of power loss in mixing systems of the
present invention that use hydraulic drive fluid were attributable
to fluid leakage past the drive ferromagnet 144 and the resulting
increased pressure drop to force the hydraulic fluid, typically
water, through the drive conduits 100 and pipes 410. Using a
pneumatic drive fluid significantly reduced pressure drop over the
drive ferromagnet 144 due to the lower viscosity of gas, typically
air, compared to water and other fluids and due to the use of seals
to reduce fluid leakage past the drive ferromagnet 144. Thus,
mixing systems of the present invention that incorporate a floating
pneumatic seal around the drive ferromagnet 144 and use lubricated
air typically have operated at 3 to 4 pounds per square inch at 5
to 6 standard liters per minute.
[0233] Four pneumatically driven mixing systems have been operated
outdoors with 6.16.+-.6% standard liters per minute of air consumed
by each (referenced at 25 degrees Celsius) under 2.75.+-.0.05
pounds per square inch. Using the standard equation for calculating
the power required to compress gas adiabatically, the minimum
energy requirement is thus 1.82 Watts/photobioreactor 310, or 0.80
Watts/m.sup.2.
[0234] Net energy usage for a commercial mixing system of the
present invention to power 240 photobioreactors 310, based on
distribution losses of 5%, a compressor of 50% efficiency and an
electrical drive of 90% efficiency yields a minimum energy
requirement of 4.26 Watts/photobioreactor 310 or 0.185
Watts/m.sup.2. An array of 240 photobioreactors 310, as illustrated
in FIG. 23, would require the use of a rotary lobe compressor rated
at 1.5 horsepower. A commercial plant with a centralized compressor
system may utilize a more efficient compressor to reduce the power
requirements. It is also possible to use bleed air from the first
compression stages of a power generating gas turbine as the
pneumatic drive fluid, which may reduce net energy cost if the
efficiency of conversion of fuel energy to electricity is
considered.
[0235] The energy required to run a pneumatically driven foil
assembly 180 is roughly equivalent to that required for a large
raceway system running at 0.25 meters per second, with an 8 inch
algae culture depth. The net power requirements for both systems
are approximately 0.2 Watts/m.sup.2. Thus, the operating expense
for both systems is $1230 per hectare per year assuming an energy
cost of $0.07 per kilowatt hour. The operating expense is higher
for small raceway systems, however. One major manufacturer of
raceway paddlewheels (Waterwheel Factory, Inc.) estimates that
motors rated for at least 20-40 Watts would be required to provide
mixing in photobioreactors 310 measuring 5 feet by 50 feet under
the most optimistic conditions, which is a factor of 5-10 times
higher power requirement than the exemplary foil mixed systems.
EXAMPLE 2
[0236] Growth of cyanobacteria in response to mixing was compared
in two reactor types that varied in mixing system design. Two
closed foil-mixed photobioreactors and two closed flume-style
raceway pond photobioreactors were tested. The oval-shaped raceway
pond photobioreactors and the foil-mixed photobioreactors were
constructed and enclosed using the same thin, flexible polymeric
film.
[0237] Inoculum cultures of a unicellular cyanobacterium were
scaled in 50-liter flat-panel culture vessels. The inoculum
cultures were then transferred into the two raceway pond
photobioreactors and the two foil-mixed photobioreactors containing
seawater and. BG-11 nutrient mix. Each raceway pond photobioreactor
contained approximately 460 liters of seawater and each foil-mixed
photobioreactor contained approximately 900 liters of seawater.
Sunlight entered each reactor across the top surface only, and the
culture depth in each reactor was 8 inches (20 centimeters),
yielding equal surface area to volume ratios for the four reactors.
The seawater in each photobioreactor was pre-filtered to 0.2-.mu.m
and had salinity of 35.
[0238] Air was delivered to each photobioreactor at a rate of 5
liters per minute, and carbon dioxide was added from 08:00 to 18:00
local standard time, controlled to a volumetric ratio of 10% carbon
dioxide to air. Each photobioreactor was maintained under ambient
irradiance and temperature conditions.
[0239] The culture contained in each foil-mixed photobioreactor was
mixed using a foil assembly comprising four foils positioned at a
depth of four inches in the algae culture contained in the
photobioreactor. Each foil had a span of seven inches and a chord
of four inches. The foils were spaced with their centerlines 14
inches apart.
[0240] The foil speed traversing the length of the foil-mixed
photobioreactor was maintained at 0.5 meters per second while the
foil was in motion. When the foil reached each end of the
photobioreactor, the motion of the foil was paused for 20 to 22
seconds in order to simulate the period of the foil, and thus
vortex reinforcement frequency, in a 50 foot-long commercial-scale,
foil-mixed photobioreactor.
[0241] Each raceway photobioreactor was operated similarly to a
paddlewheel raceway in which horizontal motion of culture was
maintained through pumping and recirculation of the flow. Rather
than using a paddlewheel, however, pumping in the raceway
photobioreactors was accomplished using four Tunze.RTM.
Turbelle.RTM. stream 6085 pumps in each bioreactor. These are
propeller pumps with a 90 mm (3.5 in.) ball design generally used
for water circulation in aquariums or tanks. Each Tunze.RTM.
Turbelle.RTM. stream 6085 produces flow rate of about 8 liters per
hour at power consumption of about 14 Watts, but the particular
power consumption of these pumps is of secondary importance.
Rather, the horizontal flow that they produced in the raceway
photobioreactor was the target. The pumps were arranged to provide
a flow rate of 0.25 m/s in an 8 inch (20 cm) deep culture in the
photobioreactor raceways. This flow rate was calculated following
Weissman et al., "Photobioreactor Design: Mixing, Carbon
Utilization, and Oxygen Accumulation," Biotechnology and
Bioengineering, Vol. 31, Pp. 336-344 (1988), equating the
electrical energy consumption of a commercial scale paddlewheel
system to generate this flow (0.21 Watts/square meter) to the power
to drive the pneumatic foil system. The power requirement was
determined using Manning's equation for hydraulic loss. Drive
efficiency was estimated as 0.31 for a paddlewheel operating at the
specified speed and depth on a 100 square meter raceway. The drive
efficiency could be as high as 0.5 for a very large system, but the
increased efficiency would not significantly increase the flow
rate, i.e., to 0.28 meters per second, under the stated power
consumption. Raceway reactors are also typically operated at mixing
speeds of 0.15-0.25 meters per second to minimize settling of algae
in the culture.
[0242] As shown in Table 1, volumetric dry weight of the
unicellular cyanobacterium in each algae culture was measured for
each photobioreactor three times per week for three weeks, as each
culture matured from growth phase to early stationary phase. Dry
weight areal biomass of the unicellular cyanobacterium in each
algal culture, shown in Table 1, was calculated based on measured
culture volumes and the surface areas of the culture in each
photobioreactor, shown in Table 2.
TABLE-US-00001 TABLE 1 Raceway Raceway Foil Mixed Foil Mixed Time
PBR 1 PBR 2 PBR 1 PBR 2 (day) mg/L g/m.sup.2 mg/L g/m.sup.2 mg/L
g/m.sup.2 mg/L g/m.sup.2 0.7 59.33 11.67 14.67 2.98 57.26 12.02
62.07 13.35 2.4 85.83 16.88 72.50 14.75 67.70 14.22 97.22 20.90 4.4
218.52 42.98 209.63 42.66 209.63 44.02 194.81 41.89 7.3 380.00
74.73 351.11 71.45 455.56 95.67 443.33 95.32 10.3 397.64 78.20
435.56 88.64 513.33 107.80 475.56 102.24 11.3 379.17 74.57 369.44
75.18 433.33 91.00 419.44 90.18 14.4 390.00 76.70 460.00 93.61
466.67 98.00 490.00 105.35 16.3 425.00 83.58 460.00 93.61 580.00
121.80 536.67 115.38 18.3 420.00 82.60 540.00 109.89 660.00 138.60
636.67 136.88 21.3 N/A N/A N/A N/A 587.73 123.42 541.67 116.46
TABLE-US-00002 TABLE 2 Reactor SA (m.sup.2) Volume (L) Raceway PBR
1 2.31 454 Raceway PBR 2 2.31 470 Foil Mixed PBR 1 4.23 889 Foil
Mixed PBR 2 4.31 926
[0243] The logistic growth model stated in Equation 2 (Kot,
"Elements of Mathematical Ecology", Cambridge University Press
(2001)) was parameterized from the areal data in each
photobioreactor as listed in Table 1.
B ( t ) = K B 0 .mu. t K + B 0 ( .mu. t - 1 ) Equation 2
##EQU00001##
[0244] wherein
[0245] t=time (day);
[0246] .mu.=specific growth rate (1/day);
[0247] B.sub.0=initial biomass (g-DW/m.sup.2); and
[0248] K=biomass abundance at stationary phase (g-DW/m.sup.2).
[0249] Values of the parameters in Equation 2 obtained from
nonlinear least square fit of the logistic growth model to the data
in Table 1 are shown in Table 3:
TABLE-US-00003 TABLE 3 Max. Growth Rate PBR .mu. K (g/m.sup.2/d)
R.sup.2 Raceway PBR 1 0.6741 79.97 13.4783 0.987 Raceway PBR 2
0.4740 96.14 11.3931 0.952 Foil Mixed PBR 1 0.4911 117.81 14.4658
0.922 Foil Mixed PBR 2 0.4307 116.90 12.5867 0.935 p value 0.3921
0.069 0.519 t-stat* 1.083 -3.618 -0.777 Significant? No Yes (90%)
No *2-tailed t-test
[0250] Plots of the average dry weight areal biomass data for each
replicate photobioreactor type in Table 1 and the parameterized
logistic growth models in Equation 2 for averaged data from both
reactor types are overlain in FIG. 43. If an exponential model of
growth is assumed instead of a logistic model and specific growth,
.mu., is calculated as ln(B.sub.2/B.sub.1)/(t.sub.2-t.sub.i), where
t.sub.1=2.4 days and t.sub.2=7.3 days, then specific growth rates
average 0.343.+-.0.055 and 0.307.+-.0.013, respectively to
foil-mixed and raceway PBRs, and are not significantly different
(t-test stat=0.8969, p=0.4644).
[0251] As shown in Table 4, both measured and modeled cumulative
biomass growth at day 16 are significantly greater in the
foil-mixed photobioreactors than in the raceway
photobioreactors:
TABLE-US-00004 TABLE 4 16-day cumulative biomass growth (g/m.sup.2)
Measured Cum. Modeled Cum. Growth (g/m.sup.2) Growth (g/m.sup.2)
Reactor Mean Std. Dev. Mean Std. Dev. Foil Mixed PBRs 118.6 4.5
116.5 1.01 Raceway PBRs 88.6 7.1 87.5 10.7 t-stat 5.0375 3.6612 p
value 0.0372 0.0672 Significant? Yes (95%) Yes (90%)
EXAMPLE 3
[0252] Capital expense is compared for conventional mixing
technology using paddlewheels to mix shallow algae cultures and for
a mixing system in accordance with the present invention using a
magnetically coupled, moving foil assembly. Details of the capital
expense for large scale paddlewheel systems have been published in
Weissman et al. Two paddlewheel systems described therein are
adapted for 0.4 hectare and 8 hectare ponds. The paddlewheel mixer
capital expense totals adjusted to present day values are
approximately $11,000 and $36,000 for the 0.4 hectare and 8 hectare
ponds, respectively. As noted in Table 5, these capital expense
totals are equivalent to approximately $53,000 and $9,000 per
hectare, respectively.
TABLE-US-00005 TABLE 5 5 .times. 50 ft reactor area (23 m.sup.2)
Open Sealed Film Component channel Photobioreactor center barrier
$116 paddlewheel $37 $37 support guard poles $2 flange $40 $40
bearings lip seals $40 paddlewheel $22 $22 shaft 3/8 SS $45 $45 end
fairings $15 gear motor $40 $40 Capex/PBR $184 $356 Capex/hectare
$80,122 $154,966 Industrial scale raceway ponds Weissmann &
Goebel 1987* Capex/hectare $52,770 (0.4 hectare) Capex/hectare
$8,850 (8 hectare) *Cost adjusted for 2011
[0253] An independent estimate for the lowest cost of a paddlewheel
mixing system suitable for use in an open 23 square meter (0.0023
hectare) raceway pond, which is equivalent to the typical size of
an enclosed bioreactor measuring 5 feet by 50 feet, was determined
by estimating the lowest material costs for a design provided by
Waterwheel Factory, Inc., a major waterwheel manufacturer. The
estimated material costs are shown in Table 2.
[0254] At $184 per raceway photobioreactor, the capital expense per
hectare for a small-scale paddlewheel mixing system is
approximately $80,000 per hectare. This estimate is plotted in FIG.
44 along with historical published data. As show by the dotted line
plotted in FIG. 44, there is a trend toward increased capital
expense per area with smaller raceway systems.
[0255] The capital expense for a foil mixing system in accordance
with the present invention used with a 23 square meter enclosed
photobioreactor is detailed in Table 6.
TABLE-US-00006 TABLE 6 feet of total Component Specification piping
lb/ft.sup.1 $/ft.sup.2 cost header piping 1.5'' SDR11 610 0.41
0.492 $300 (central row) header piping 1.25'' 1420 0.16 0.192 $273
(sides) SDR15.3 mixer tubing 1/2'' 12,480 0.048 0.0576 $719 SDR10.1
quantity $/item compressor 1.5 hp rotary 1 1000 $1,000 lobe blower
electronics ABB 1 350 $350 and control VFD or 4-way valve external
NdFeB 240 6.4 $1,536 magnets Grade N42 1.25''OD .times. 0.75'' ID
.times. 1/8'' internal NdFeB 480 2.37 $1,138 magnets Grade N40
0.5''OD .times. 0.25'' ID .times. 1/4'' mixer.sup.3 2 lbs HDPE 240
4.38 $1,051 cost/ $6,366 module.sup.4 cost/ $27 PBR.sup.5 cost/
$11,533 hectare .sup.1Performance Pipe IPS size data for PE 4710.
.sup.2$1.20/lb in accord with pricing from Ferguson Enterpries,
Inc. .sup.3Cost calculated by 3X cost of plastic for standard blow
molding, $0.73/lb in accord with ICIS pricing. .sup.41 module
consists of 240 photobioreactors arranged as in FIG. 23. .sup.5Each
photobioreactor is 5 feet wide by 50 feet long and has a wet area
of 23 square meters.
[0256] The capital expense illustrated in Table 6 includes the
drive and distribution components of the pneumatic chive foil
mixing system for a set of 240 photobioreactors (in 4 rows of 60).
Here, the capital expense per bioreactor is $27, or approximately
$11,500 per hectare. Thus, the capital expense per area for a
facility constructed using photobioreactor modules of this size is
independent of the size of the facility.
[0257] Capital expense is compared for foil mixing systems and
raceway mixing systems in Table 7 and FIG. 44.
TABLE-US-00007 TABLE 7 individual reactor size (hectare)
Capex/hectare open paddlewheel 8 8,848 Weissman&Goebel (1987)
open paddlewheel 0.4 52,772 Weissman&Goebel (1987) open channel
0.0023 80,122 paddlewheel estimate sealed film PBR 0.0023 154,966
paddlewheel estimate foil mixer 0.0023 11,533
[0258] The data in Table 7 and FIG. 44 show that the currently
sized foil mixing system is approximately one order of magnitude
less expensive than raceway paddlewheel mixing systems on an areal
basis, except for raceways which approach a size of 8 hectares.
Thus, foil mixing reduces the capital cost of mixing on all scales
which can be enclosed at reasonable expense. Even if the cost to
enclose a 0.4 hectare (approximately 1 acre) pond was negligible,
foil mixing systems are still 4.5 times less expensive than raceway
paddlewheel mixing systems. The cost for a sealed raceway system
would exceed the estimates shown in Table 6 due to the cost of
seals, guards, central barriers and end pieces that would need to
be manufactured for the photobioreactors.
[0259] The foregoing Examples 1 through 3 demonstrate that the
biomass output of a foil mixed system was comparable or better than
a raceway pond, while the operating expense of the foil system was
equivalent or lower. The most distinctive advantage of the foil
mixed system is its low capital expense, which makes up a large
fraction of the total cost even when spread over a 15 year
operating period as shown in Table 8. Capital expense is a much
larger fraction of the total expense for the paddlewheel raceway
systems for areas that are practical to enclose (e.g. 23 square
meters). Thus the total cost of mixing to generate biomass is much
higher in the paddlewheel system, by a factor as large as 8. Here
the cost of mixing is only about 4 cents per kilogram of dry
weight, while cost of mixing for an enclosed paddlewheel system
could be 34 cents per kilogram.
TABLE-US-00008 TABLE 8 Pneumatically driven Enclosed foil mixed
paddlewheel Annual expenditure/hectare reactors (23 sq. m) reactors
(23 sq. m) Capex/15 years.sup.1 769 10,331 Opex 1,230 .sup.(2)5,775
Total 1,999 16,106 Biomass yield per year.sup.3 47,450 47,450 Total
cost.sup.4 0.042 0.339 ($/kg of biomass) .sup.1Interest on loan not
considered in either case. .sup.(2)20 Watts per photobioreactor as
the minimum suggested by Waterwheel Factory, Inc. for 0.25 meters
per second flow. .sup.3Assuming maximum growth rate (13 grams per
square meter per day) demonstrated in Example 2 is sustained over a
year. .sup.4Considering only the cost of mixing.
[0260] In certain embodiments, as illustrated in FIGS. 46, 47 and
48, the depth of the algae culture is shallow and a crossbar 590 is
used to create a shallow breaking wave front 600 on the surface 320
of the algae culture. In certain embodiments, the depth of the
algae culture is preferably approximately 1 to 2 inches. In certain
embodiments, the depth of the algae culture is preferably
approximately 1 inch. The crossbar 590 is attached to a follower
element 110 that is disposed on a drive conduit 100. The crossbar
590 is propelled using magnetic coupling between the follower
element 110 and a drive element 190, to which pneumatic or
hydraulic motive force is applied.
[0261] The crossbar 590 is propelled at sufficient speed to
displace fluid along the length of the photobioreactor 310, such
that a wave front 600 is generated that moves at a higher speed
than the shallow water wave in the direction of travel of the
crossbar 590. For an algae culture of 1 inch depth, a wave front
600 may be generated by a crossbar traveling at 0.5 meters per
second.
[0262] The wave front 600 generated at the surface 320 of the algae
culture provides enhanced mass transfer between the liquid algae
culture and the air above the surface 320 of the algae culture.
Dissolved oxygen content in the algae culture would be lower when a
breaking wave front 600 is generated than when a breaking wave
front 600 is not generated in the algae culture. The wave front 600
also generates provides vertical mixing of the algae within the
culture, which enhances photosynthetic productivity and diffusion
of nutrients.
[0263] The range of traverse, or stroke length, of the crossbar 590
may be substantially shorter than the length of the photobioreactor
310. Stroke length can be controlled by placing stoppers inside the
drive conduit 100 to restrict the motion of the drive element 190
or around the outside of the drive conduit 100 to restrict the
motion of the follower element 110. In one embodiment, a stroke
length of 1 meter is sufficient to generate a wave front 600 that
can propagate for 80% of the length of the photobioreactor 310.
Shorter stroke length of the wave front 600 produces shorter
propagation distance of the wave front 600.
[0264] A wave front 600 can be created in a photobioreactor 310
that is 50 feet long using 8.5-9.5 pounds per square inch hydraulic
motive force in a drive conduit 100 of 0.52 inches inside diameter,
which propels the crossbar 590 at 0.33-0.5 meters per second. If
the crossbar 590 is located in the center of the photobioreactor
310, a cycle time of approximately 10 seconds for the crossbar 590
to complete one oscillation allows for one breaking wave front 600
to be present on either side of the crossbar 590 at any time, while
maintaining low energy usage, approximately 4 Watts per
photobioreactor 310, equivalent to a mixing system using a foil
assembly 180.
[0265] The crossbar 590 used to generate a wave front 600 may be
buoyant and may have chamfered edges on the lower surfaces of the
crossbar 590 to generate lift, which prevents the crossbar 590 from
touching the bottom of the photobioreactor 310 and reduces friction
and wear on the components of the crossbar 590 and the
photobioreactor 310. The crossbar 590 will be optimally designed
such that reflections of the wave front 600 are minimized. The wave
front 600 preferably dissipates when it reaches the end of the
photobioreactor 310 and does not disrupt the motion of the crossbar
590. Resonant operation of the crossbar 590 is possible but may be
difficult to control at low capital expense.
[0266] Multiple crossbars 590 in one photobioreactor 310 can be
driven in the same manner as a system using multiple foil
assemblies 180. In combination with reducing the stroke length of
each crossbar 590, a configuration employing multiple crossbars 590
can be used to generate wave fronts 600 at higher frequencies, so
that more than two breaking wave fronts 600 would be present in the
photobioreactor 310 at any time.
[0267] Although the present invention has been described in
considerable detail with reference to certain embodiments thereof,
other embodiments are possible. Therefore, the spirit and scope of
the appended claims should not be limited to the description of the
embodiments. contained therein.
* * * * *